Fix memory leak in FlutterSwitchSemanticsObject (#28065)

diff --git a/shell/platform/darwin/ios/framework/Source/SemanticsObject.mm b/shell/platform/darwin/ios/framework/Source/SemanticsObject.mm
index 6d30c34..50ff757 100644
--- a/shell/platform/darwin/ios/framework/Source/SemanticsObject.mm
+++ b/shell/platform/darwin/ios/framework/Source/SemanticsObject.mm
@@ -100,7 +100,7 @@
                            uid:(int32_t)uid {
   self = [super initWithBridge:bridge uid:uid];
   if (self) {
-    _nativeSwitch = [[[UISwitch alloc] init] retain];
+    _nativeSwitch = [[UISwitch alloc] init];
   }
   return self;
 }