gjs: remove obsolete gobject-introspection build input

Since b2f9cd34e7 ("gobject-introspection: use wrapper.nix for the
native package too so we can propagate the dev output"),
gobject-introspection doesn't need to be included in buildInputs when
it's in nativeBuildInputs, as it's propagated from the setup hook.

Removing the build input fixes evaluation in buildPackages when
cross-compiling to platforms that gobject-introspection is unsupported
on, like ghcjs.

Tested building natively on x86_64-linux, in
pkgsCross.aarch64-multiplatform, and in pkgsCross.ghcjs.buildPackages.

pkgs/development/libraries/gjs/default.nix

@@ -27,7 +27,6 @@
 
 let
   testDeps = [
-    gobject-introspection # for Gio and cairo typelibs
     gtk3 atk pango.out gdk-pixbuf harfbuzz
   ];
 in stdenv.mkDerivation rec {