lib: Indicate whether notmuch_database_remove_message removed anything.

lib: Indicate whether notmuch_database_remove_message removed anything.

Similar to the return value of notmuch_database_add_message, we now
enhance the return value of notmuch_database_remove_message to
indicate whether the message document was entirely removed (SUCCESS)
or whether only this filename was removed and the document exists
under other filenamed (DUPLICATE_MESSAGE_ID).