Remove TARGET_GEN_MEMSET_SCRATCH_RTX since it's not used anymore.

The target hook is only used by i386, and the current definition is
same as default gen_reg_rtx.

gcc/ChangeLog:

	* builtins.cc (builtin_memset_read_str): Replace
	targetm.gen_memset_scratch_rtx with gen_reg_rtx.
	(builtin_memset_gen_str): Ditto.
	* config/i386/i386-expand.cc
	(ix86_convert_const_wide_int_to_broadcast): Replace
	ix86_gen_scratch_sse_rtx with gen_reg_rtx.
	(ix86_expand_vector_move): Ditto.
	* config/i386/i386-protos.h (ix86_gen_scratch_sse_rtx):
	Removed.
	* config/i386/i386.cc (ix86_gen_scratch_sse_rtx): Removed.
	(TARGET_GEN_MEMSET_SCRATCH_RTX): Removed.
	* doc/tm.texi: Remove TARGET_GEN_MEMSET_SCRATCH_RTX.
	* doc/tm.texi.in: Ditto.
	* target.def: Ditto.