Revert "bugfix: use of LuaJIT FFI when disabling the Lua code cache would lead to LuaJIT GC assertion failures like "g->gc.total == sizeof(GG_State)" because we unloaded the standard "ffi" module. thanks Ron Gomes for reporting this issue."

Preventing unloading "ffi" does not reallys solve the issue here because
we can end up with FFI ctype redefinition errors.

This reverts commit b3218be26612282109897651ed5ccd4ab18a9e06.