Remove no-longer-used SSTATUS_VS_MASK

Spike removed this in
https://github.com/riscv-software-src/riscv-isa-sim/commit/60243a3bf9f86de5b8b58807ae218f1e3aedc31c

I checked other uses of encoding.h (riscv-pk, riscv-test-env,
riscv-openocd), and confirmed that none of them use this value.