[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>

gcc/
	* config/rs6000/rs6000-builtins.def: Rephrase
	to remove RS6000_BTC_SPECIAL from comment.
	* config/rs6000/rs6000.h (RS6000_BTC_UNARY, RS6000_BTC_BINARY,
	RS6000_BTC_TERNARY, RS6000_BTC_QUATERNARY,
	RS6000_BTC_QUINARY, RS6000_BTC_SENARY, RS6000_BTC_OPND_MASK,
	RS6000_BTC_SPECIAL, RS6000_BTC_PREDICATE, RS6000_BTC_ABS,
	RS6000_BTC_DST, RS6000_BTC_TYPE_MASK, RS6000_BTC_MISC,
	RS6000_BTC_CONST, RS6000_BTC_PURE, RS6000_BTC_FP,
	RS6000_BTC_QUAD, RS6000_BTC_PAIR, RS6000_BTC_QUADPAIR,
	RS6000_BTC_ATTR_MASK, RS6000_BTC_SPR, RS6000_BTC_VOID,
	RS6000_BTC_CR, RS6000_BTC_OVERLOADED, RS6000_BTC_GIMPLE,
	RS6000_BTC_MISC_MASK, RS6000_BTC_MEM, RS6000_BTC_SAT,
	RS6000_BTM_ALWAYS): Delete.