lib/classifier: Make classifier_remove() more robust.

classifier already provides lockless lookups, and protected
modifications.  When user wants to remove a flow, we currently require
the flow to exist in the classifier.  To be thread safe, this requires
the caller to introduce their own mutex, lock it before a lookup, and
then issue classifier_remove() while the lock is still held.

This patch relaxes the "existence requirement" of the rule in
classifier_remove(), allowing it to be called on a rule that may have
already been removed from the classifier.  This allows users to do a
classifier_lookup() and classifier_remove() without additional
syncronization.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>