runtime: remove unused scheduler knob

Blockingsyscall was used in net package on windows,
it's not used anymore.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12436043

src/pkg/runtime/cgocall.c

@@ -98,15 +98,6 @@ static void unwindm(void);
 static void endcgo(void);
 static FuncVal endcgoV = { endcgo };
 
-// Gives a hint that the next syscall
-// executed by the current goroutine will block.
-// Currently used only on windows.
-void
-net·runtime_blockingSyscallHint(void)
-{
-	g->blockingsyscall = true;
-}
-
 void
 runtime·cgocall(void (*fn)(void*), void *arg)
 {
@@ -158,11 +149,7 @@ runtime·cgocall(void (*fn)(void*), void *arg)
 	 * so it is safe to call while "in a system call", outside
 	 * the $GOMAXPROCS accounting.
 	 */
-	if(g->blockingsyscall) {
-		g->blockingsyscall = false;
-		runtime·entersyscallblock();
-	} else
-		runtime·entersyscall();
+	runtime·entersyscall();
 	runtime·asmcgocall(fn, arg);
 	runtime·exitsyscall();
 

src/pkg/runtime/runtime.h

@@ -269,7 +269,6 @@ struct	G
 	bool	ispanic;
 	bool	issystem;	// do not output in stack dump
 	bool	isbackground;	// ignore in deadlock detector
-	bool	blockingsyscall;	// hint that the next syscall will block
 	bool	preempt;	// preemption signal, duplicates stackguard0 = StackPreempt
 	int8	raceignore;	// ignore race detection events
 	M*	m;		// for debuggers, but offset not hard-coded
@@ -313,7 +312,6 @@ struct	M
 	int32	dying;
 	int32	profilehz;
 	int32	helpgc;
-	bool	blockingsyscall;
 	bool	spinning;
 	uint32	fastrand;
 	uint64	ncgocall;	// number of cgo calls in total