branch.py: Always check out the remote wmf/<branch>

do_core_work():

Previously mediawiki/core's master branch would be checked out, then
we'd make a local wmf/<branch> branch from there. Now the remove
wmf/<branch> is checked out immediately.

Noted a bug in the "Remove all existing submodules" section of the code.
That section should probably be removed entirely.

Made a few tweaks to make the function idempotent (i.e., re-runnable without
bad behavior).

Bug: T373709