Cleanup switchToConfig + bug fix in removeInterwiki

* env.switchToConfig was a bit convoluted -- cleaned it up.

  Also added error checking for missing prefix and unavailable
  API uri.

  Removed hack from ConfigRequest.

* ParsoidConfig.removeInterwiki had edge case bugs -- this is
  currently unused, so no harm done.

Change-Id: I6fa40c9f4cf4101d041fb0cf584266874d95d199
Merge "Cleanup switchToConfig + bug fix in removeInterwiki"