RISC-V: Remove the unimplemented extensions.

RISC-V: Remove the unimplemented extensions.

Although spec had defined and ratified p, v and n extensions,
but we don't have any related implementaitons so far, so keep
them in the supported extension table looks weird.  Remove them
until we have the related implementations.

	opcodes/
	* riscv-opc.c (riscv_ext_version_table): Remove the p, v, n
	and their versions.