RISC-V: Remove duplicate `#include "riscv-vector-switch.def"`

Hi,

This patch remove the duplicate `#include "riscv-vector-switch.def"` statement
and add #undef for ENTRY and TUPLE_ENTRY macros later.

Best,
Lehua

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (struct mode_vtype_group): Remove duplicate
	#include.
	(ENTRY): Undef.
	(TUPLE_ENTRY): Undef.