Reland "Remove exception for relative CIPD"

This is a reland of 2d29261e

Original change's description:
> Remove exception for relative CIPD
> 
> This seems to no longer be necessary, as relative paths 'just work' for
> CIPD deps now.
> 
> Change-Id: I3843548dc74d29af90e0c402f0459dbe60934423
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1529132
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Reviewed-by: John Budorick <jbudorick@chromium.org>
> Commit-Queue: Brandon Tolsch <btolsch@chromium.org>

Change-Id: Idac857ead961587e7295c9be6d53c578448eb6b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1532268Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Brandon Tolsch <btolsch@chromium.org>

gclient.py

@@ -1878,10 +1878,6 @@ class CipdDependency(Dependency):
         should_recurse=False,
         relative=relative,
         condition=condition)
-    if relative:
-      # TODO(jbudorick): Implement relative if necessary.
-      raise gclient_utils.Error(
-          'Relative CIPD dependencies are not currently supported.')
     self._cipd_package = None
     self._cipd_root = cipd_root
     # CIPD wants /-separated paths, even on Windows.