Remove the disabling of layer constants for GLES2

The GLSL shader generation supports layer combine constants so there's
no need to disable it for GLES2. It looks like there was also code for
it in the GLES2 wrapper so I'm not sure why it was disabled in the
first place.

cogl/cogl-blend-string.c

@@ -185,16 +185,6 @@ validate_tex_combine_statements (CoglBlendStringStatement *statements,
                              "not texture combining";
               goto error;
             }
-#ifdef HAVE_COGL_GLES2
-          if (arg->source.info->type == COGL_BLEND_STRING_COLOR_SOURCE_CONSTANT)
-            {
-              error_string = "Using a constant for texture combining isn't "
-                             "currently supported with GLES 2 "
-                             "(TODO: glTexEnvf)";
-              detail = COGL_BLEND_STRING_ERROR_GPU_UNSUPPORTED_ERROR;
-              goto error;
-            }
-#endif
         }
     }