rs6000, remove built-ins __builtin_vsx_vperm_8hi and __builtin_vsx_vperm_8hi_uns

rs6000, remove built-ins __builtin_vsx_vperm_8hi and __builtin_vsx_vperm_8hi_uns

The two built-ins __builtin_vsx_vperm_8hi and __builtin_vsx_vperm_8hi_uns
are redundant. The are covered by the overloaded vec_perm built-in.  The
built-ins are not documented and do not have test cases.

The removal of these built-ins was missed in commit gcc r15-1923 on
7/9/2024.

This patch removes the redundant built-ins.

gcc/ChangeLog:
	* config/rs6000/rs6000-builtins.def (__builtin_vsx_vperm_8hi,
	__builtin_vsx_vperm_8hi_uns): Remove built-in definitions.