glsl: remove now unused varying linker code

All of this varying linking code has now been ported to the NIR linker.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25371>

src/compiler/glsl/shader_cache.cpp

@@ -54,7 +54,6 @@
 #include "ir_rvalue_visitor.h"
 #include "ir_uniform.h"
 #include "linker.h"
-#include "link_varyings.h"
 #include "nir.h"
 #include "program.h"
 #include "serialize.h"