libmtp: Remove local libmtp repo from manifest.

libmtp is now built from upstream+patches, so the local libmtp repo can be
dereferenced.

BUG=chromium:958691
TEST=dry-run cq

Change-Id: I5d9639af183af6385528b45ea1af4996ae76b1ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/manifest/+/1837016Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Anand Mistry <amistry@chromium.org>
Tested-by: Andrew McRae <amcrae@chromium.org>
Commit-Queue: Andrew McRae <amcrae@chromium.org>

full.xml

@@ -450,9 +450,6 @@ Your sources have been sync'd successfully.
            name="chromium/llvm-project/cfe/tools/clang-format"
            remote="chromium"
            groups="minilayout,buildtools,labtools" />
-  <project path="src/chromium/src/third_party/libmtp"
-           name="chromium/deps/libmtp"
-           remote="chromium" />
   <project path="src/chromium/src/third_party/Python-Markdown"
            name="chromium/src/third_party/Python-Markdown"
            remote="chromium" />