pw-apply: remove mbox.o before mb2q normalization

mb2q --mboxout, apparently, will append new contents to existing mbox.o
instead of overwriting the file. This leads to dangerous and confusing
behavior if there are local mbox.o leftovers.

Make sure that old mbox.o is gone before we do normalization.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>