MediaLibrary: Simplify setExternalLibvlcInstance
MediaLibrary: Simplify setExternalLibvlcInstance

Don't force a parser recreation when replacing libvlc instance since the
instance isn't cached anymore. The next run will simply use the updated
instance
refs #361

chouquette/medialibrary,chouquette/medialibrary,chouquette/medialibrary,chouquette/medialibrary