Revert left-overs from previous plan9 arm64 port commit

Author: rafael2k

src/syscall/pwd_plan9.go

@@ -29,8 +29,6 @@ var (
 // rescheduling on a different thread (potentially with a different
 // working directory) before the syscall is executed.
 func Fixwd() {
-	runtime.LockOSThread()
-	defer runtime.UnlockOSThread()
 	wdmu.Lock()
 	defer wdmu.Unlock()
 	fixwdLocked()
@@ -74,8 +72,6 @@ func getwd() (wd string, err error) {
 }
 
 func Getwd() (wd string, err error) {
-	runtime.LockOSThread()
-	defer runtime.UnlockOSThread()
 	wdmu.Lock()
 	defer wdmu.Unlock()