Previously we implemented two ways to get a pty for lxc-attach:
	1. get a pty in the container
	2. get a pty on the host

Where 1. was the default and 2. was only tried after 1. failed.
For safety and simplicity reasons, we remove 1. and just keep 2. around.
Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>