gyp: Cleanup Aura-related files and dependencies.

* Stop manually excluding `_aura.cc` files from the build, this is done
  automatically by Chromium's `filename_rules.gypi`. Ditto for files
  ending in `_views.cc`.

* Remove the dependency on the `aura_test_support` and `wm` targets. Not
  only does depending on a test target even when no tests are being
  bring a ton of additional dependencies, but neither targets seem to be
  needed at all, and were probably only needed in an earlier version of
  commit 336c03a ("runtime: Add aura-only native support").