classifier: Refactor interface for classifier_remove().

classifier: Refactor interface for classifier_remove().

Until now, classifier_remove() returned either null or the classifier rule
passed to it, which is an unusual interface.  This commit changes it to
return true if it succeeds or false on failure.

In addition, most of classifier_remove()'s callers know ahead of time that
it must succeed, even though most of them didn't bother with an assertion,
so this commit adds a classifier_remove_assert() function as a helper.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Tested-by: Yifeng Sun <pkusunyifeng@gmail.com>
Reviewed-by: Yifeng Sun <pkusunyifeng@gmail.com>