refactor: Remove unused variable "tapTimes" from multi_touch_tap_dete…

…ctor_test.dart (#2379)

This is a cleanup identified on this issue: #2308
Using an amazing unused-code tooling
Now, since Flame is a public API, unused code might not be trivial - it might just mean untested code.

This one was a no-brainer -- the variable was clearly replaced by specific counters during the writing of the test and forgotten there:

  int nOnTapDown = 0;
  int nOnLongTapDown = 0;
  int nOnTapUp = 0;
  int nOnTap = 0;
  int nOnTapCancel = 0;

It wasn't even increment and the taps are already thoroughly tested more granularly

packages/flame/test/events/game_mixins/multi_touch_tap_detector_test.dart

@@ -88,7 +88,6 @@ void main() {
 }
 
 class _GameWithMultiTouchTapDetector extends Game with MultiTouchTapDetector {
-  int tapTimes = 0;
   bool updated = false;
   bool rendered = false;