notmuch: Implement search-tags as an alias for "search --output=tags *"

notmuch: Implement search-tags as an alias for "search --output=tags *"

Ever since we added support for "notmuch search --output=tags" the
"notmuch search-tags" command has been redundant. The recent addition
of alias support makes it easy to drop the explicit search-tags
command in favor of a simple alias that runs "notmuch search
--output=tags *".

So there's no longer any documentation of the search-tags command, but
existing scripts will not break at all.