spirv: Drop the void *ptr from vtn_value

It isn't being used for anything.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5278>
diff --git a/src/compiler/spirv/vtn_private.h b/src/compiler/spirv/vtn_private.h
index b3a915d..5cd5a46 100644
--- a/src/compiler/spirv/vtn_private.h
+++ b/src/compiler/spirv/vtn_private.h
@@ -582,7 +582,6 @@
    struct vtn_decoration *decoration;
    struct vtn_type *type;
    union {
-      void *ptr;
       char *str;
       nir_constant *constant;
       struct vtn_pointer *pointer;