runtime: remove unused mid function

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/10036047

src/pkg/runtime/extern.go

@@ -112,9 +112,6 @@ func (f *Func) FileLine(pc uintptr) (file string, line int) {
 // implemented in symtab.c
 func funcline_go(*Func, uintptr) (string, int)
 
-// mid returns the current OS thread (m) id.
-func mid() uint32
-
 // SetFinalizer sets the finalizer associated with x to f.
 // When the garbage collector finds an unreachable block
 // with an associated finalizer, it clears the association and runs

src/pkg/runtime/proc.c

@@ -1732,14 +1732,6 @@ runtime·golockedOSThread(bool ret)
 	FLUSH(&ret);
 }
 
-// for testing of wire, unwire
-void
-runtime·mid(uint32 ret)
-{
-	ret = m->id;
-	FLUSH(&ret);
-}
-
 void
 runtime·NumGoroutine(intgo ret)
 {