Do not support new installations via .php5 entrypoint any longer

Continues the path of deprecating $wgScriptExtension and all of
the related baggage.

You'll only end up with sadness when we rip out of the rest of
this later so don't let people shoot themselves in the foot.

Change-Id: I34cd1d2c266405ebc761a271e3740e972fb7cf2f