Remove dead and broken code from gulpfile (chartjs#5794)

In commit 3a4550c, the task unittestWatch was removed. However, the watch task links to it, when executed with the test flag. As watching the unittest is possible with `gulp unittest --watch`, this code is not needed anymore and thus removed.

gulpfile.js

@@ -233,9 +233,6 @@ function moduleSizesTask() {
 }
 
 function watchTask() {
-  if (util.env.test) {
-    return gulp.watch('./src/**', ['build', 'unittest', 'unittestWatch']);
-  }
   return gulp.watch('./src/**', ['build']);
 }