[RISCV] Remove RISCVSubtarget::getArchMinVLen()/getArchMaxVLen().

Fold them into their only callers, getRealMinVLen()/getArchMaxVLen().

It is unclear right now when these are needed so removing to discourage
misuse.

Between Zvl*b extensions, vector length command line options, and
vscale range, we have several ways to influence vector length. We
need to try to keep all code on the same page.