Don't clear the realm cache after performing a migration

AFAICT it's never needed (since RLMRealms with custom schema aren't cached, it
shouldn't be possible to have a cached RLMRealm for a path that needs a
migration), and removing completely unrelated paths from the cache is actively
bad.
Merge pull request #1203 from realm/tg-migration-no-clear-cache

Don't clear the realm cache after performing a migration