runtime: delete unused isscanstatus function

Change-Id: I693250e980cc60ea151736b42ac6b1426ab801b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/171765
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/proc.go

@@ -692,13 +692,6 @@ func freezetheworld() {
 	usleep(1000)
 }
 
-func isscanstatus(status uint32) bool {
-	if status == _Gscan {
-		throw("isscanstatus: Bad status Gscan")
-	}
-	return status&_Gscan == _Gscan
-}
-
 // All reads and writes of g's status go through readgstatus, casgstatus
 // castogscanstatus, casfrom_Gscanstatus.
 //go:nosplit