regexp/syntax: remove InstLast

This was added by the one-pass CL (post Go 1.2)
so it can still be removed.

Removing because surely there will be new operations
added later, and we can't change the constant value
once we define it, so "last" is a bad concept to expose.

Nothing uses it.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/81160043

src/pkg/regexp/syntax/prog.go

@@ -36,7 +36,6 @@ const (
 	InstRune1
 	InstRuneAny
 	InstRuneAnyNotNL
-	InstLast
 )
 
 var instOpNames = []string{
@@ -54,7 +53,7 @@ var instOpNames = []string{
 }
 
 func (i InstOp) String() string {
-	if i >= InstLast {
+	if uint(i) >= uint(len(instOpNames)) {
 		return ""
 	}
 	return instOpNames[i]