GRAILS-11296: When page removed from pageCache, clear metaClass

When GroovyPageMetaInfo is removed from the pageCache, remove the metaclass from the pageClass in the GroovyPageMetaInfo.
Merge branch '2.2.x' of https://github.com/candrews/grails-core into candrews-2.2.x
Merge branch '2.3.x' of github.com:grails/grails-core into 2.3.x