Remove extraneous newlines from presubmit output

Prior to this CL, we have:

```
Running Python 2 presubmit upload checks ...

Python 2 presubmit checks passed.
Running Python 3 presubmit upload checks ...

Presubmit checks took 10.2s to calculate.

Python 3 presubmit checks passed.
 ios/chrome/browser/... | 20 +++++++++-----------
...
```

The first two checks are harder to parse with the newlines. This CL removes
them:

```
Running Python 2 presubmit upload checks ...
Python 2 presubmit checks passed.

Running Python 3 presubmit upload checks ...
Presubmit checks took 10.2s to calculate.
Python 3 presubmit checks passed.

 ios/chrome/browser/... | 20 +++++++++-----------
...
```

Change-Id: Iaf7ac4b203c401f60d74884fd9bdd3ba4ee12c20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3512873Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Louis Romero <lpromero@chromium.org>

presubmit_support.py

@@ -1731,7 +1731,6 @@ def DoPresubmitChecks(change,
       else:
         messages.setdefault('Messages', []).append(result)
 
-    sys.stdout.write('\n')
     for name, items in messages.items():
       sys.stdout.write('** Presubmit %s **\n' % name)
       for item in items:
@@ -1741,10 +1740,10 @@ def DoPresubmitChecks(change,
     total_time = time_time() - start_time
     if total_time > 1.0:
       sys.stdout.write(
-          'Presubmit checks took %.1fs to calculate.\n\n' % total_time)
+          'Presubmit checks took %.1fs to calculate.\n' % total_time)
 
     if not should_prompt and not presubmits_failed:
-      sys.stdout.write('%s presubmit checks passed.\n' % python_version)
+      sys.stdout.write('%s presubmit checks passed.\n\n' % python_version)
     elif should_prompt and not presubmits_failed:
       sys.stdout.write('There were %s presubmit warnings. ' % python_version)
       if may_prompt:

tests/presubmit_unittest.py

@@ -902,7 +902,6 @@ def CheckChangeOnCommit(input_api, output_api):
       text = (
           RUNNING_PY_CHECKS_TEXT + 'Warning, no PRESUBMIT.py found.\n'
           'Running default presubmit script.\n'
-          '\n'
           '** Presubmit ERRORS **\n!!\n\n'
           'Was the presubmit check useful? If not, run "git cl presubmit -v"\n'
           'to figure out which PRESUBMIT.py was run, then run git blame\n'