Remove Nix leftovers from Gtk process launcher

Since commit 974037a7 Nix uses the Efl process launcher and workqueue,
but there were still some leftovers in GTK process launcher, mainly
the WEB_PROCESS_CMD_PREFIX approach to prefixing a program to WebProcess.

This commit removes the additions we had in the GTK launcher.

Reviewed by: Trust me, I'm an engineer.