configure.ac: Remove option to build without libffi
configure.ac: Remove option to build without libffi

We've always had a hard dependency on libffi headers.
Commit 5798f94b6a727b93 added a direct module dependency on libffi
instead of indirect via GI. Remove the option to build without libffi.

GNOME/pygobject,pexip/pygobject,pexip/pygobject,pexip/pygobject,GNOME/pygobject,sfeltman/pygobject,sfeltman/pygobject,sfeltman/pygobject,GNOME/pygobject