terminal: remove sigwinch command

terminal: remove sigwinch command

SIGWINCH is handled in lxc_terminal_signalfd_cb().

I cannot for the life of me figure out what this is supposed to do.
Afaict, it scans a global list that is totally unnecessary and also
let's say you have 100 ttys and for a single one SIGWINCH is sent. In
that case the whole list is walked and two ioctl()s are performed: one
to get window size one to set window size. For 99 of them the window
size hasn't changed.
If we see issues we can revert!

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>