RISC-V: Remove unnecessary vread_csr/vwrite_csr intrinsic.

According to below RVV doc, the related intrinsic is not longer needed.

https://github.com/riscv-non-isa/rvv-intrinsic-doc/pull/249



Signed-off-by: Pan Li <pan2.li@intel.com>

gcc/ChangeLog:

	* config/riscv/riscv_vector.h (enum RVV_CSR): Removed.
	(vread_csr): Ditto.
	(vwrite_csr): Ditto.