Scriptrunner doesn't clean up MultiParentClassLoader on plugin enabled event

Scriptrunner doesn't clean up MultiParentClassLoader on plugin enabled event.
MultiParentClassLoader stores result of plugin classloader resolution for @WithPlugin annotation. It got reset on plugin disabled event, but not on plugin enabled event.

In case when WithPlugin annotation was used before referenced plugin is enabled, unsuccessful result of resolution is stored and it should be cleared if corresponding plugin is enabled, to allow MultiParentClassLoader attempt resolution again.