Remove unused FIRFirestore instances dictionary (#2937)

This was copied into FSTFirestoreComponent but never removed.

Author: wilhuff

Firestore/Source/API/FIRFirestore.mm

@@ -72,45 +72,6 @@ @implementation FIRFirestore {
   FIRFirestoreSettings *_settings;
 }
 
-+ (NSMutableDictionary<NSString *, FIRFirestore *> *)instances {
-  static dispatch_once_t token = 0;
-  static NSMutableDictionary<NSString *, FIRFirestore *> *instances;
-  dispatch_once(&token, ^{
-    instances = [NSMutableDictionary dictionary];
-  });
-  return instances;
-}
-
-+ (void)initialize {
-  NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
-  [center addObserverForName:kFIRAppDeleteNotification
-                      object:nil
-                       queue:nil
-                  usingBlock:^(NSNotification *_Nonnull note) {
-                    NSString *appName = note.userInfo[kFIRAppNameKey];
-                    if (appName == nil) return;
-
-                    NSMutableDictionary *instances = [self instances];
-                    @synchronized(instances) {
-                      // Since the key for instances isn't just the app name, iterate over all the
-                      // keys to get the one(s) we have to delete. There could be multiple in case
-                      // the user calls firestoreForApp:database:.
-                      NSMutableArray *keysToDelete = [[NSMutableArray alloc] init];
-                      NSString *keyPrefix = [NSString stringWithFormat:@"%@|", appName];
-                      for (NSString *key in instances.allKeys) {
-                        if ([key hasPrefix:keyPrefix]) {
-                          [keysToDelete addObject:key];
-                        }
-                      }
-
-                      // Loop through the keys found and delete them from the stored instances.
-                      for (NSString *key in keysToDelete) {
-                        [instances removeObjectForKey:key];
-                      }
-                    }
-                  }];
-}
-
 + (instancetype)firestore {
   FIRApp *app = [FIRApp defaultApp];
   if (!app) {