libical: 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/libical/default.nix

@@ -59,7 +59,6 @@ stdenv.mkDerivation rec {
     glib
     libxml2
     icu
-    gobject-introspection
   ];
 
   cmakeFlags = [