Forget cache entry for maybeBroadcastWatch when unwatching.

Inspired by @kamilkisiela's comment:
#7149 (comment)

Should fix #7149 and #7086.

src/cache/inmemory/inMemoryCache.ts

@@ -181,6 +181,11 @@ export class InMemoryCache extends ApolloCache<NormalizedCacheObject> {
     }
     return () => {
       this.watches.delete(watch);
+      this.watchDep.dirty(watch);
+      // Remove this watch from the LRU cache managed by the
+      // maybeBroadcastWatch OptimisticWrapperFunction, to prevent memory
+      // leaks involving the closure of watch.callback.
+      this.maybeBroadcastWatch.forget(watch);
     };
   }