glib: remove util-linuxMinimal assertion when building for Linux

According to 868eb83019, this assertion was introduced when glib
started requiring this dependency by default due to libmount. However,
it still supports building without libmount and without this dependency
on Linux and this assertion prohibits doing this using overrides.

This changes fixes that and does not affect the builds output.

```
% nix-build -A glib
[...]
/nix/store/c6p0n1xsd7zj2dnxm2d4m85nx5fgbjbj-glib-2.88.3-bin
% git checkout HEAD^
[...]
% nix-build -A glib
/nix/store/c6p0n1xsd7zj2dnxm2d4m85nx5fgbjbj-glib-2.88.3-bin
%
```

pkgs/by-name/gl/glib/package.nix

@@ -23,7 +23,7 @@
   docutils,
   gi-docgen,
   # use util-linuxMinimal to avoid circular dependency (util-linux, systemd, glib)
-  util-linuxMinimal ? null,
+  util-linuxMinimal,
   buildPackages,
 
   # this is just for tests (not in the closure of any regular package)
@@ -43,8 +43,6 @@
     && stdenv.hostPlatform.isLittleEndian == stdenv.buildPlatform.isLittleEndian,
 }:
 
-assert stdenv.hostPlatform.isLinux -> util-linuxMinimal != null;
-
 let
   glib-untested = glib.overrideAttrs { doCheck = false; };
   # break dependency cycles