cmd/compile: cull UINF

It was used with Node.Ullman, which is now gone.

Change-Id: I83b167645659ae7ef70043b7915d642e42ca524f
Reviewed-on: https://go-review.googlesource.com/37761
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/go.go

@@ -12,7 +12,6 @@ import (
 )
 
 const (
-	UINF            = 100
 	BADWIDTH        = -1000000000
 	MaxStackVarSize = 10 * 1024 * 1024
 )