'Removes the bookmark for the block.
Arguments:
usage_key: UsageKey of the block.
Returns:
Bool indicating whether the bookmark was removed.'

def unset_bookmarked(self, usage_key):

    try:
        api.delete_bookmark(self._user, usage_key=usage_key)
    except ObjectDoesNotExist:
        log.error(u'Bookmark    with    usage_id:    %s    does    not    exist.', usage_key)
        return False
    bookmarks_cache = self._bookmarks_cache(usage_key.course_key)
    if (bookmarks_cache is not None):
        deleted_bookmark_index = None
        usage_id = unicode(usage_key)
        for (index, bookmark) in enumerate(bookmarks_cache):
            if (bookmark['usage_id'] == usage_id):
                deleted_bookmark_index = index
                break
        if (deleted_bookmark_index is not None):
            bookmarks_cache.pop(deleted_bookmark_index)
    return True
