mesa: Remove _mesa_pack_rgba_span_float and tmp_pack.h

mesa: Remove _mesa_pack_rgba_span_float and tmp_pack.h

_mesa_pack_rgba_span_float was the last of the color span functions
and we have replaced all calls to it with calls to _mesa_format_convert,
so we can remove it together with tmp_pack.h which was used to
generate the pack functions for multiple types that were used from
the various color span functions that have been removed.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>