i965: Delete the fabulous target_to_target() function.

i965: Delete the fabulous target_to_target() function.

gl_texture_object's Target field is never a cube face enumeration, so
target_to_target is just the identity function.  Aptly named, at least.

I verified this by putting an assert(!"ZOMG, CUBES!") in the cube face
case, and running Piglit.  Nothing ever hit it.  Beyond that, I
inspected the code in mesa/main.

This could probably also be deleted from i915, but I haven't tested
there.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>