cmd/compile/internal/ir: remove old variable capture logic

cmd/compile/internal/ir: remove old variable capture logic

This logic is no longer used, since the removal of the non-unified
frontends.

Change-Id: I7256d4eb06f0b4e5308bc7c7476e90af9e70b7d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/524215
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>