mesa: remove unused vertex attrib WEIGHT

We don't support ARB_vertex_blend.

Note that the attribute aliasing check for ARB_vertex_program had to be
rewritten.

vbo_context: 20344 -> 20008 bytes
gl_context: 74672 -> 74616 bytes

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/main/arrayobj.c

@@ -277,9 +277,6 @@ _mesa_initialize_vao(struct gl_context *ctx,
    /* Init the individual arrays */
    for (i = 0; i < ARRAY_SIZE(vao->VertexAttrib); i++) {
       switch (i) {
-      case VERT_ATTRIB_WEIGHT:
-         init_array(ctx, vao, VERT_ATTRIB_WEIGHT, 1, GL_FLOAT);
-         break;
       case VERT_ATTRIB_NORMAL:
          init_array(ctx, vao, VERT_ATTRIB_NORMAL, 3, GL_FLOAT);
          break;