Remove unused 'singleStepping' state on Process

We don't care, at the process level, whether or not we're single
stepping. That state is really only relevant at the thread level.

proc/proc.go

@@ -40,7 +40,6 @@ type Process struct {
 	frameEntries            frame.FrameDescriptionEntries
 	lineInfo                *line.DebugLineInfo
 	firstStart              bool
-	singleStepping          bool
 	os                      *OSProcessDetails
 	arch                    Arch
 	ast                     *source.Searcher
@@ -386,8 +385,6 @@ func (dbp *Process) Continue() error {
 // Single step, will execute a single instruction.
 func (dbp *Process) Step() (err error) {
 	fn := func() error {
-		dbp.singleStepping = true
-		defer func() { dbp.singleStepping = false }()
 		for _, th := range dbp.Threads {
 			if th.blocked() {
 				continue

proc/proc_darwin.go

@@ -282,6 +282,7 @@ func (dbp *Process) trapWait(pid int) (*Thread, error) {
 			}
 			dbp.exited = true
 			return nil, ProcessExitedError{Pid: dbp.Pid, Status: status.ExitStatus()}
+
 		case C.MACH_RCV_INTERRUPTED:
 			if !dbp.halt {
 				// Call trapWait again, it seems
@@ -290,6 +291,7 @@ func (dbp *Process) trapWait(pid int) (*Thread, error) {
 				continue
 			}
 			return nil, nil
+
 		case 0:
 			return nil, fmt.Errorf("error while waiting for task")
 		}
@@ -305,7 +307,7 @@ func (dbp *Process) trapWait(pid int) (*Thread, error) {
 					return dbp.Threads[int(port)], nil
 				}
 				th := dbp.Threads[int(port)]
-				if dbp.firstStart || dbp.singleStepping || th.singleStepping {
+				if dbp.firstStart || th.singleStepping {
 					dbp.firstStart = false
 					return dbp.Threads[int(port)], nil
 				}