genxword: remove obsolete gobject-introspection build input

Since b2f9cd34e75 ("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/applications/misc/genxword/default.nix

···
   ];
 
   buildInputs = [
-    gobject-introspection
     pango
     gtksourceview3
   ];