Allow to remove API callbacks
Allow to remove API callbacks

Change API to allow to remove a given callback URL and to remove all
callbacks for a given ApiImplementor. Also, remove the callbacks when
the implementor is removed.

psiinon/zaproxy,meitar/zaproxy,kingthorin/zaproxy,thc202/zaproxy,kingthorin/zaproxy,thc202/zaproxy,psiinon/zaproxy,zaproxy/zaproxy,Ali-Razmjoo/zaproxy,Ali-Razmjoo/zaproxy,meitar/zaproxy,zapbot/zaproxy,zapbot/zaproxy,zaproxy/zaproxy,kingthorin/zaproxy,gmaran23/zaproxy,meitar/zaproxy,meitar/zaproxy,kingthorin/zaproxy,meitar/zaproxy,psiinon/zaproxy,gmaran23/zaproxy,zapbot/zaproxy,zapbot/zaproxy,psiinon/zaproxy,Ali-Razmjoo/zaproxy,gmaran23/zaproxy,kingthorin/zaproxy,meitar/zaproxy,zapbot/zaproxy,meitar/zaproxy,zapbot/zaproxy,zaproxy/zaproxy,thc202/zaproxy,gmaran23/zaproxy,psiinon/zaproxy,thc202/zaproxy,thc202/zaproxy,gmaran23/zaproxy,meitar/zaproxy,zaproxy/zaproxy,kingthorin/zaproxy,thc202/zaproxy,kingthorin/zaproxy,zaproxy/zaproxy,gmaran23/zaproxy,Ali-Razmjoo/zaproxy,zaproxy/zaproxy,psiinon/zaproxy,zaproxy/zaproxy,gmaran23/zaproxy,Ali-Razmjoo/zaproxy,Ali-Razmjoo/zaproxy,psiinon/zaproxy,thc202/zaproxy,Ali-Razmjoo/zaproxy,zapbot/zaproxy