[RISCV][NFC] Simplify the vector pipe names in SiFive7 sched model (#181268)

Instead of creating a `VA` for single vector pipe configuration (e.g.
X280) and `VA1` + `VA2` for dual vector pipes ones (e.g. X390), we could
have just use `VA1` in the former case to simplify the related name
aliases.

NFC.

GitOrigin-RevId: 77efa4af205f29cd4c5f73e896becaa830f3d337