[PATCH, rs6000] Remove the (no longer used) BTC defines.
These defines are no longer used once the rs6000 built-in
reworks were completed. Time to remove them.
There was a reference to RS6000_BTC_SPECIAL in a TODO comment
in rs6000-builtins.def. That comment remains, but I have updated
the comment to refer to "SPECIAL" processing, instead of having it
refer directly to the _BTC_SPECIAL macro.
2022-05-18 Will Schmidt <will_schmidt@vnet.ibm.com>