Delete ieee128_float_type_node.

Delete ieee128_float_type_node.

Now that we always use float128_type_node for __float128, we can delete the tree
variable ieee128_float_type_node.

2022-08-12   Michael Meissner  <meissner@linux.ibm.com>

gcc/

	* config/rs6000/rs6000-builtin.cc (rs6000_type_string): Delete
	ieee128_float_type_node.
	* config/rs6000/rs6000-c.cc (rs6000_target_modify_macros): Likewise.
	(rs6000_cpu_cpp_builtins): Likewise.
	* config/rs6000/rs6000.cc (rs6000_translate_mode_attribute): Likewise.
	* config/rs6000/rs6000.h (RS6000_BTI_ieee128_float): Delete.
	(ieee128_float_type_node): Delete.