Withdraw support for the DECEDM escape sequence.

Having decided that the terminal's local echo setting shouldn't be
allowed to propagate through to termios, I think the local edit
setting shouldn't either. Also, no other terminal emulator I know
seems to implement this sequence, and if you enable it, things get
very confused in general. I think it's generally better off absent; if
somebody turns out to have been using it, then we'll at least be able
to find out what it's good for.

unix/uxplink.c

@@ -79,10 +79,6 @@ char *x_get_default(const char *key)
 {
     return NULL;		       /* this is a stub */
 }
-bool term_ldisc(Terminal *term, int mode)
-{
-    return false;
-}
 static void plink_echoedit_update(Seat *seat, bool echo, bool edit)
 {
     /* Update stdin read mode to reflect changes in line discipline. */