proc: remove proc_tty_ldisc variable

/proc/tty/ldisc appear to be unused as a directory and
it had been always that way.

But it is userspace visible thing.

Cowardly remove only in-kernel variable holding it.

[akpm@linux-foundation.org: add comment]
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>