runtime: remove the started field from sweepdata

This bool doesn't seem to be used anymore. Remove it.

Change-Id: Ic73346a98513c392d89482c5e1d818a90d713516
Reviewed-on: https://go-review.googlesource.com/c/go/+/419654
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/runtime/mgcsweep.go

@@ -33,10 +33,9 @@ var sweep sweepdata
 
 // State of background sweep.
 type sweepdata struct {
-	lock    mutex
-	g       *g
-	parked  bool
-	started bool
+	lock   mutex
+	g      *g
+	parked bool
 
 	nbgsweep    uint32
 	npausesweep uint32