maintenance/dev: Clean up router.php

maintenance/dev: Clean up router.php

* Remove code paths that were unreachable for me.

* Remove display_errors/error_reporting overrides.
  These can be set as-needed from LocalSettings.php if desired,
  and similar (but slightly different) settings are already set
  by DevelopmentSettings.php.

* Default to text/plain instead of letting PHP handle it.
  This makes makes the result more predictable, instead of varying
  by PHP version and underlying OS behaviour. Anything we want to
  support is already in our 'mime.types' file, and ours is a
  superset of PHP's anyway.

Change-Id: I46608ebd8e225642cfeb804ec32739c2fcbd4d25