pan/midgard: Remove pack_color define

pan/midgard: Remove pack_color define

Unused at the moment.

../src/panfrost/midgard/midgard_compile.c:124:29: warning: ‘m_pack_colour’ defined but not used [-Wunused-function]
  124 |  static midgard_instruction m_##name(unsigned ssa, unsigned address) { \
      |                             ^~
../src/panfrost/midgard/midgard_compile.c:145:22: note: in expansion of macro ‘M_LOAD_STORE’
  145 | #define M_LOAD(name) M_LOAD_STORE(name, false)
      |                      ^~~~~~~~~~~~
../src/panfrost/midgard/midgard_compile.c:213:1: note: in expansion of macro ‘M_LOAD’
  213 | M_LOAD(pack_colour);
      | ^~~~~~

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3543>