RISC-V: Remove @ of vec_duplicate pattern

RISC-V: Remove @ of vec_duplicate pattern

It's obvious the @ of vec_duplicate pattern is duplicate.

Regression passed.

Committed.
gcc/ChangeLog:

	* config/riscv/riscv-v.cc (gen_const_vector_dup): Use global expand function.
	* config/riscv/vector.md (@vec_duplicate<mode>): Remove @.
	(vec_duplicate<mode>): Ditto.