emacs: tag deleted face bugfix

emacs: tag deleted face bugfix

Commit d25d33ff cleaned up some of the tag face code. However, for the
face notmuch-tag-deleted it used the test

((class color) (supports :strike-through))

to decide whether to use red strikethrough or inverse-video (emacs in
a terminal typically doesn't support red strikethrough, but in X it does).

However, it seems that test often returns true even though red
strikethrough is not supported. This breaks the tag update code -- the
wrong thing is displayed to the user.

Thus we make the test explicitly more specific, changing the test to

((class color) (supports :strike-through "red"))