[RISCV] Delete dead elideCopy code in InsertVSETVLI [nfc]

This code should be dead. A simple whole register copy of an IMPLICIT_DEF, is simply an IMPLICIT_DEF of it's own. (This would not be true for freeze, but is for copy.)  If we find a case which gets here with vector operand copy of an IMPLICIT_DEF, we most likely have an earlier missed optimization anyways.  (The most recent case of this was e6c7a3a5, found by Craig during review of this patch.)  There might be others, and if so, we'll revisit them individually as regressions are reported.

Differential Revision: https://reviews.llvm.org/D127996