remove git cache unlock command

It's never been called since 2023 according to the metric

Bug: 339231299
Change-Id: I6de76058f8daa00eb1115eabf04f915c332ef79f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5533622
Auto-Submit: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>

git_cache.py

@@ -890,13 +890,6 @@ def CMDfetch(parser, args):
     return 0
 
 
-@subcommand.usage('do not use - it is a noop.')
-@metrics.collector.collect_metrics('git cache unlock')
-def CMDunlock(parser, args):
-    """This command does nothing."""
-    print('This command does nothing and will be removed in the future.')
-
-
 class OptionParser(optparse.OptionParser):
     """Wrapper class for OptionParser to handle global options."""
     def __init__(self, *args, **kwargs):