Remove left-overs of WIDEST_HARDWARE_FP_SIZE

This removes the single remaining use of WIDEST_HARDWARE_FP_SIZE in GCC,
which has been anectodal for a long time, and all the associated bits.

gcc/
	* config/aarch64/aarch64.h (WIDEST_HARDWARE_FP_SIZE): Delete.
	* config/alpha/alpha.h (WIDEST_HARDWARE_FP_SIZE): Likewise.
	* config/i386/i386.h (WIDEST_HARDWARE_FP_SIZE): Likewise.
	* config/ia64/vms.h (WIDEST_HARDWARE_FP_SIZE): Likewise.
	* config/pa/pa.h (WIDEST_HARDWARE_FP_SIZE): Likewise.
	* config/rs6000/rs6000.h (WIDEST_HARDWARE_FP_SIZE): Likewise.
	* config/s390/s390.h (WIDEST_HARDWARE_FP_SIZE): Likewise.
	* config/sparc/sparc.h (WIDEST_HARDWARE_FP_SIZE): Likewise.
	* config/visium/visium.h (WIDEST_HARDWARE_FP_SIZE): Likewise.
	* doc/tm.texi.in (WIDEST_HARDWARE_FP_SIZE): Likewise.
	* doc/tm.texi: Regenerate.
	* system.h (WIDEST_HARDWARE_FP_SIZE): Poison.

gcc/ada/
	* gcc-interface/decl.cc (gnat_to_gnu_entity): Use the precision of
	longest_float_type_node to compute the maximum size of FP types.