[RISCV] Remove unnecessary check for RVE from determineCalleeSaves. NFCI

The SavedRegs BitVector is checks against the CSR list later. We have
a separate CSR list for RVE that excludes X16-31 so we don't need
to filter here.

If it was needed, it would be needed for the next block of code too
which didn't have an RVE check.