mesa: fix new gcc6 warnings

mesa: fix new gcc6 warnings

src/mesa/main/texstore.c:92:22: warning: ‘map_1032’ defined but not used [-Wunused-const-variable]
 static const GLubyte map_1032[6] = { 1, 0, 3, 2, ZERO, ONE };
                      ^~~~~~~~
src/mesa/main/texstore.c:91:22: warning: ‘map_3210’ defined but not used [-Wunused-const-variable]
 static const GLubyte map_3210[6] = { 3, 2, 1, 0, ZERO, ONE };
                      ^~~~~~~~
src/mesa/main/texstore.c:90:22: warning: ‘map_identity’ defined but not used [-Wunused-const-variable]
 static const GLubyte map_identity[6] = { 0, 1, 2, 3, ZERO, ONE };
                      ^~~~~~~~~~~~

These appear to be unused since:

commit 8ec6534b266549cdc2798e2523bf6753924f6cde
Author:     Iago Toral Quiroga <itoral@igalia.com>
AuthorDate: Wed Oct 15 13:42:11 2014 +0200

    mesa: Use _mesa_format_convert to implement texstore_rgba.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>