Remove temporary workaround for E3 entry-point migration

This removes the if/else inclusion of either E3Experiments.php or
Experiments.php as the entry point for the E3Experiments extension which I had
put in place in I01deec11b to facilitate a smooth migration from the latter to
the former.

Change-Id: I95bc0d850b297bcfde2bf7265bb09e0854a63773