[Clang][RISCV] Remove forced-sw-shadow-stack (#115355)
This option was used to override the behavior of
`-fsanitize=shadowcallstack` on RISC-V backend, which by default use a
hardware implementation if possible, to use the software implementation
instead. After #112477 and #112478, now two implementation
is represented by independent options and we no longer need it.
