[RISCV] Remove riscv-v-fixed-length-vector-elen-max command line option.

[RISCV] Remove riscv-v-fixed-length-vector-elen-max command line option.

This was added before Zve extensions were defined. I think users
should use Zve32x or Zve32f now. Though we will lose support for limiting
ELEN to 16 or 8, but I hope no one was using that.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D123418