cmd/compile: remove outdated comment in prove.go

cmd/compile: remove outdated comment in prove.go

See the cases above all of theses are implemented (except XOR which has a blank case with comments).

Change-Id: I9e2994490dac89e86ba70c1abeb1af1cbcf032e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/604416
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>