glsl: remove duplicate embedded struct validation

Commit c98deb18d5 in 2010 disallowed embedded struct definitions
in ES. Then in 2013 d9bb8b7b56 disallowed it for everything but
GLSL 1.10.

Commit c98deb18d5 seemed the cleanest way to do the check so its
been extended to cover GL and the other version has been removed.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

src/compiler/glsl/glsl_parser_extras.h

@@ -289,13 +289,6 @@ struct _mesa_glsl_parse_state {
    unsigned forced_language_version;
    gl_shader_stage stage;
 
-   /**
-    * Number of nested struct_specifier levels
-    *
-    * Outside a struct_specifier, this is zero.
-    */
-   unsigned struct_specifier_depth;
-
    /**
     * Default uniform layout qualifiers tracked during parsing.
     * Currently affects uniform blocks and uniform buffer variables in