Add support for deleting a shared preference (#1018)

Summary:
This change makes it possible to remove preferences. I also added a `Delete` context menu option to `DataInspector` because I needed it to implement this feature. passy confirmed that it makes sense to add this because delete is a common action.

Fixes https://github.com/facebook/flipper/issues/451
Pull Request resolved: https://github.com/facebook/flipper/pull/1018

Reviewed By: jknoxville

Differential Revision: D21086308

Pulled By: passy

fbshipit-source-id: 551ff0908d5e6c93f58d6012b42e1ee3531de997

iOS/Plugins/FlipperKitUserDefaultsPlugin/FKUserDefaultsPlugin.m

@@ -82,6 +82,15 @@ static NSString* const kAppSuiteUserDefaultsName = @"App Suite UserDefaults";
                                     forKey:preferenceName];
               [responder success:[sharedPreferences dictionaryRepresentation]];
             }];
+
+  [connection receive:@"deleteSharedPreference"
+            withBlock:^(NSDictionary* params, id<FlipperResponder> responder) {
+              NSUserDefaults* sharedPreferences =
+                  [self sharedPreferencesForParams:params];
+              NSString* preferenceName = params[@"preferenceName"];
+              [sharedPreferences removeObjectForKey:preferenceName];
+              [responder success:[sharedPreferences dictionaryRepresentation]];
+            }];
 }
 
 - (void)didDisconnect {