runtime: remove unused g.readyg field

Commit 0e6a6c51 removed readyExecute a long time ago, but left behind
the g.readyg field that was used by readyExecute. Remove this now
unused field.

Change-Id: I41b87ad2b427974d256ec7a7f6d4bdc2ce8a13bb
Reviewed-on: https://go-review.googlesource.com/13111Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/runtime2.go

@@ -257,7 +257,6 @@ type g struct {
 	startpc        uintptr // pc of goroutine function
 	racectx        uintptr
 	waiting        *sudog // sudog structures this g is waiting on (that have a valid elem ptr)
-	readyg         *g     // scratch for readyExecute
 
 	// Per-G gcController state
 	gcalloc    uintptr // bytes allocated during this GC cycle