Fix memory leak in FlutterSwitchSemanticsObject

[ColdPaleLight]

Both methods alloc and retain will increase the reference count of _nativeSwitch,so its reference count is 2

_nativeSwitch = [[[UISwitch alloc] init] retain];

After the dealloc method is called, reference count of _nativeSwitch is 1 instead of 0, so it will not be destroyed

- (void)dealloc {
  [_nativeSwitch release];
  [super dealloc];
}

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[flutter-dashboard]

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[chinmaygarde]

I guess this leak was introduced in a832502eaf