Streamline the Karma configuration to remove obsolete settings and make
it easier to see what non-default configuration we have supplied.

 - Remove `grep` option handling. The `--grep` option when running tests
   is now used in gulpfile.js to select which _files_ are included in
   the test.

 - Remove comments which add little value

 - Remove config settings that just set default values which are not
   expected to change in future