Web process extension must always create a custom script world

This code is currently unreachable beacuse guid is always sent when
initializing the web process. But if it were not unreachable, it would
be a major security issue, because Epiphany internal APIs would be
exposed to the web. So let's remove this.

I think this was maybe a leftover from an earlier draft of the
WebExtension branch that perhaps attempted to use a separate
EphyWebProcessExtension for each WebExtension? Regardless, it's not
going to work.
3 jobs for mcatanzaro/webextension-guid in 17 minutes and 21 seconds (queued for 1 second)
Status Job ID Name Coverage
  .Pre
passed #1127380
check-code-style

00:00:40

 
  Test
passed #1127381
flatpak
flatpak

00:16:41

passed #1127382
flatpak
flatpak scanbuild

00:14:14