[RISCV] Drop single letter b extension support

It splited into several zb* extensions, and `b` is dropped after
0.93, so it time to retired that as other non-ratified zb* extensions.

Currntly clang can accept that with warning:

$ clang -target riscv64-elf ~/hello.c -S  -march=rv64gcb
'+b' is not a recognized feature for this target (ignoring feature)
'+b' is not a recognized feature for this target (ignoring feature)
'+b' is not a recognized feature for this target (ignoring feature)

Reviewed By: asb, luismarques

Differential Revision: https://reviews.llvm.org/D136812