[RISCV] Cleanup setOperationAction for ISD::BITCAST with Zfa and D extension. NFC

We only need Custom handling for i64 on RV32. This will be used by
type legalization. We don't need to make it custom for f64 to get
type legalization to custom split i64.

If f64 and i64 are legal types, then ISD::BITCAST should be legal.