runtime: remove unused debug.wbshadow

The wbshadow implementation was removed a year and a half ago in
1635ab7d, but the GODEBUG setting remained. Remove the GODEBUG
setting since it doesn't do anything.

Change-Id: I19cde324a79472aff60acb5cc9f7d4aa86c0c0ed
Reviewed-on: https://go-review.googlesource.com/36618
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/runtime1.go

@@ -329,7 +329,6 @@ var debug struct {
 	scavenge          int32
 	scheddetail       int32
 	schedtrace        int32
-	wbshadow          int32
 }
 
 var dbgvars = []dbgVar{
@@ -349,7 +348,6 @@ var dbgvars = []dbgVar{
 	{"scavenge", &debug.scavenge},
 	{"scheddetail", &debug.scheddetail},
 	{"schedtrace", &debug.schedtrace},
-	{"wbshadow", &debug.wbshadow},
 }
 
 func parsedebugvars() {