Revert ".gitignore: ignore *.cover and *.mbx"

This reverts commit 534066a9.

It's actively detrimental in that it hides files that shouldn't be
hidden.

If I have some b4 mbx file in my git directory, it either was already
applied with "git am" and is now stale, or maybe it's waiting for that
to happen.  In neither case is "ignore it" the right option.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>