RISC-V: Remove unsupported isa string info print

RISC-V: Remove unsupported isa string info print

/proc/cpuinfo should just print all the isa string as an information
instead of determining what is supported or not. ELF hwcap can be
used by the userspace to figure out that.

Simplify the isa string printing by removing the unsupported isa string
print and all related code.

The relevant discussion can be found at
http://lists.infradead.org/pipermail/linux-riscv/2019-September/006702.html

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: http://lists.infradead.org/pipermail/linux-riscv/2019-September/006702.html
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>