[BUG #7270] don't lint-check each demo in Demobrowser when building

- lint-checking the demos when building slowed down the build process considerably
  (like 10x)
- mostly, the framework classes that go into each demo where lint-checked over
  and over again
- for Demobrowser it should be enough to only check with an explicit 'generate.py
  lint', which also includes all demos