nvidia-x11: revert "add an assert that useSettings implies more than libsOnly"

THe change caused nixGL instantiation failures:

- https://github.com/nix-community/nixGL/issues/157
- https://github.com/nix-community/nixGL/issues/154

I missed the fact that there is no easy way to oberride `useSettings` as
it's an internal argument as ollosed to `useLibs`.

Instead of fixing it let's revert it back and try again later.

This reverts commit 9c51fb0606.

pkgs/os-specific/linux/nvidia-x11/generic.nix

@@ -60,7 +60,6 @@
 
 with lib;
 
-assert useSettings -> !libsOnly;
 assert !libsOnly -> kernel != null;
 assert versionOlder version "391" -> sha256_32bit != null;
 assert useSettings -> settingsSha256 != null;