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 
Reviewed-by: Michael Mortensen 
diff --git a/docs/manifest-format.md b/docs/manifest-format.md
index b439dcb..676e031 100644
--- a/docs/manifest-format.md
+++ b/docs/manifest-format.md
@@ -396,10 +396,4 @@
 Manifest files stored in `$TOP_DIR/.repo/local_manifests/*.xml` will
 be loaded in alphabetical order.
 
-Additional remotes and projects may also be added through a local
-manifest, stored in `$TOP_DIR/.repo/local_manifest.xml`. This method
-is deprecated in favor of using multiple manifest files as mentioned
-above.
-
-If `$TOP_DIR/.repo/local_manifest.xml` exists, it will be loaded before
-any manifest files stored in `$TOP_DIR/.repo/local_manifests/*.xml`.
+The legacy `$TOP_DIR/.repo/local_manifest.xml` path is no longer supported.