devpts: simplify two-level sysctl registration for pty_kern_table

There is no need to declare two tables to just create directories,
this can be easily be done with a prefix path with register_sysctl().

Simplify this registration.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>