runtime: prune some dead variables

Change-Id: I7a1c3079b433c4e30d72fb7d59f9594e0d5efe47
Reviewed-on: https://go-review.googlesource.com/16178
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/runtime/proc.go

@@ -336,7 +336,6 @@ func allgadd(gp *g) {
 
 	lock(&allglock)
 	allgs = append(allgs, gp)
-	allg = &allgs[0]
 	allglen = uintptr(len(allgs))
 	unlock(&allglock)
 }

src/runtime/runtime2.go

@@ -612,16 +612,12 @@ const (
 
 var (
 	emptystring string
-	allg        **g
 	allglen     uintptr
-	lastg       *g
 	allm        *m
 	allp        [_MaxGomaxprocs + 1]*p
 	gomaxprocs  int32
 	panicking   uint32
-	goos        *int8
 	ncpu        int32
-	signote     note
 	forcegc     forcegcstate
 	sched       schedt
 	newprocs    int32