libcontainer to manage /dev and /proc (#4945)

libcontainer already manages `/dev`, overriding task_dir - so let's use it for `/proc` as well and remove deadcode.

drivers/shared/executor/executor_linux.go

@@ -496,6 +496,12 @@ func configureIsolation(cfg *lconfigs.Config, command *ExecCommand) {
 			Flags:       syscall.MS_NOSUID | syscall.MS_STRICTATIME,
 			Data:        "mode=755",
 		},
+		{
+			Source:      "proc",
+			Destination: "/proc",
+			Device:      "proc",
+			Flags:       defaultMountFlags,
+		},
 		{
 			Source:      "devpts",
 			Destination: "/dev/pts",