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>
diff --git a/subcmds/manifest.py b/subcmds/manifest.py
index fb020d8..f0a0d06 100644
--- a/subcmds/manifest.py
+++ b/subcmds/manifest.py
@@ -30,7 +30,7 @@
   _helpDescription = """
 
 With the -o option, exports the current manifest for inspection.
-The manifest and (if present) local_manifest.xml are combined
+The manifest and (if present) local_manifests/ are combined
 together to produce a single manifest file.  This file can be stored
 in a Git repository for use during future 'repo init' invocations.