RISC-V: Remove the removed single-letter extensions

There were a few single-letter extensions that we had references to
floating around in the kernel, but that never ended up as actual ISA
specs and have mostly been replaced by multi-letter extensions.  This
removes the references to those extensions.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231110175903.2631-1-palmer@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>