patman: Remove the patch-management code

Delete the command-line tool and its supporting modules, now that this
functionality lives in the standalone patch-manager package.

Keep the modules that buildman still imports (commit and patchstream,
plus their dependencies series, get_maintainer and settings), along with
the stub command. Trim __init__.py to match.

Signed-off-by: Simon Glass <sjg@chromium.org>