cmd/compile/internal/ir: remove unused Name.{Offset,SetOffset}

cmd/compile/internal/ir: remove unused Name.{Offset,SetOffset}

These aren't used and don't do anything useful anyway.

Change-Id: I2865f6bbb0409fa59b1fde32abb7c5f81a6bdb7c
Reviewed-on: https://go-review.googlesource.com/c/go/+/516195
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>