Fix Remote Config travis flakes (#3738)

Author: paulb777

FirebaseRemoteConfig/Sources/RCNConfigDBManager.m

@@ -286,7 +286,6 @@ - (void)removeDatabaseOnDatabaseQueueAtPath:(NSString *)path {
     if (!strongSelf) {
       return;
     }
-    RCN_MUST_NOT_BE_MAIN_THREAD();
     if (sqlite3_close(strongSelf->_database) != SQLITE_OK) {
       [self logDatabaseError];
     }

FirebaseRemoteConfig/Tests/Unit/RCNRemoteConfigTest.m

@@ -224,8 +224,7 @@ - (void)setUp {
 }
 
 - (void)tearDown {
-  // Causes crash if main thread exits before the RCNConfigDB queue cleans up
-  //  [_DBManager removeDatabaseOnDatabaseQueueAtPath:_DBPath];
+  [_DBManager removeDatabaseOnDatabaseQueueAtPath:_DBPath];
   [[NSUserDefaults standardUserDefaults] removePersistentDomainForName:_userDefaultsSuiteName];
   [super tearDown];
 }