Remove procStart

It's never used and not needed. Our pipe is created with
syscall.SOCK_CLOEXEC, so pipe will be closed once container
process executed successfully, parent process will read EOF
and continue. If container process got error before executed,
we'll write procError to sync with parent.

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>

libcontainer/process_linux.go

@@ -247,8 +247,6 @@ loop:
 			return newSystemError(err)
 		}
 		switch procSync.Type {
-		case procStart:
-			break loop
 		case procReady:
 			if err := p.manager.Set(p.config.Config); err != nil {
 				return newSystemError(err)