largefiles: handle that a found standin file doesn't exist when removing it
I somehow ended up in a situation where hg crashed on an unlink I introduced in
328545c7d8a1.
I don't know how it happened and can't reproduce it. It seems like it only can
happen when the file is removed between the time of check in a working
directory context walk that finds a standin file, and the time of use when we
try to remove it because the corresponding largefile doesn't exist.
But better safe than sorry: replace the plain unlink with unlinkpath with
ignoremissing=True. That will also remove remaining empty directories, which
arguably is more correct.