manifest: drop support for local_manifest.xml

We deprecated this 8 years ago.  Time to drop it to simplify the code
as it'll help with refactoring in this module to not migrate it.

Change-Id: I2deae5496d1f66a4491408fcdc95cd527062f8b6
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/280798
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Michael Mortensen <mmortensen@google.com>