monotouch: cleanup callback variable when unsub
monotouch: cleanup callback variable when unsub

grokify/c-sharp,grokify/c-sharp,grokify/c-sharp,marceloinacio/c-sharp,marceloinacio/c-sharp,marceloinacio/c-sharp,marceloinacio/c-sharp,grokify/c-sharp,grokify/c-sharp,marceloinacio/c-sharp