Revert "Add previous_got_revision and previous_got_revision_cp to output"

This reverts commit 6d0c0ffe.

Reason for revert: this appears to  break builders if reference
is a commit hash:

git log --format=%B -n1 HEAD failed with code 128

Original change's description:
> Add previous_got_revision and previous_got_revision_cp to output
>
> As chrome browser infra works to improve CQ times, we would like to
> easily glance at builds and see how many new commits were checked out.
> Currently you have to open up bot_update stdout to see the previous
> HEAD revision and go from there. Having this conveniently outputted
> would be very convenient.
>
> If the bot has previously checked out the repo, output which revision
> and commit position were checked out.
>
> example LED build showing new output: https://ci.chromium.org/swarming/task/51592e46d6a70e10?server=chromium-swarm.appspot.com
>
> Change-Id: Ib8a6f152a593446ce4082477bc9ada44f1fd1fc3
> Bug: 1170568
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2644829
> Reviewed-by: Josip Sokcevic <sokcevic@google.com>
> Commit-Queue: Stephanie Kim <kimstephanie@google.com>

TBR=infra-scoped@luci-project-accounts.iam.gserviceaccount.com,sokcevic@google.com,kimstephanie@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1170568
Change-Id: I50d6ef6802c2eb1614cca804391c7826a79a2cf9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2657023Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -839,8 +839,7 @@ def get_commit_position(git_path, revision='HEAD'):
   return None
 
 
-def parse_got_revision(gclient_output, got_revision_mapping,
-                       prev_checkout_info):
+def parse_got_revision(gclient_output, got_revision_mapping):
   """Translate git gclient revision mapping to build properties."""
   properties = {}
   solutions_output = {
@@ -863,16 +862,9 @@ def parse_got_revision(gclient_output, got_revision_mapping,
       revision = git('rev-parse', 'HEAD', cwd=dir_name).strip()
       commit_position = get_commit_position(dir_name)
 
-    prev_revision, prev_cp = prev_checkout_info.get(dir_name.rstrip('/'),
-                                                    (None, None))
-
     properties[property_name] = revision
-    if prev_revision:
-      properties['prev_%s' % property_name] = prev_revision
     if commit_position:
       properties['%s_cp' % property_name] = commit_position
-    if prev_cp:
-      properties['prev_%s_cp' % property_name] = prev_cp
 
   return properties
 
@@ -1113,16 +1105,6 @@ def prepare(options, git_slns, active):
   print('Fetching Git checkout at %s@%s' % (first_sln, revisions[first_sln]))
   return revisions, step_text
 
-def current_checkout_info(solutions):
-  r = {}
-  build_dir = os.getcwd()
-
-  for sln in solutions:
-    sln_dir = path.join(build_dir, sln['name'])
-    if path.isdir(sln_dir) and not is_broken_repo_dir(sln_dir):
-      r[sln['name']] = (git('rev-parse', 'HEAD', cwd=sln_dir).strip(),
-                        get_commit_position(sln_dir))
-  return r
 
 def checkout(options, git_slns, specs, revisions, step_text):
   print('Using Python version: %s' % (sys.version,))
@@ -1146,8 +1128,6 @@ def checkout(options, git_slns, specs, revisions, step_text):
     # create file, no content
     pass
 
-  prev_checkout_info = current_checkout_info(git_slns)
-
   should_delete_dirty_file = False
 
   try:
@@ -1221,8 +1201,7 @@ def checkout(options, git_slns, specs, revisions, step_text):
   if not revision_mapping:
     revision_mapping['got_revision'] = first_sln
 
-  got_revisions = parse_got_revision(gclient_output, revision_mapping,
-                                     prev_checkout_info)
+  got_revisions = parse_got_revision(gclient_output, revision_mapping)
 
   if not got_revisions:
     # TODO(hinoka): We should probably bail out here, but in the interest