[RISCV] Simplify fixed-vector-fp.ll run lines. NFC

This removes the different scalar fp16 configurations i.e. zfh and
zfhmin, since all these ops should be able to be lowered without
scalarizing.

This ends up revealing a couple of cases where we end up scalarizing
unexpectedly, e.g. sqrt/fabs/round with v6f16 and zvfhmin.

It also removes the zvl256b configurations, since I couldn't find
anything that specifically needed to test this.

Author: lukel97