kill TIOCSERGSTRUCT

Once upon a time a bunch of serial drivers used to provide that;
today it's only amiserial and it's FUBAR - the structure being
copied to userland includes kernel pointers, fields with
config-dependent size, etc.  No userland code using it could
possibly survive - e.g. enabling lockdep definitely changes the
layout.  Besides, it's a massive infoleak.

Kill it.  If somebody needs that data for debugging purposes, they
can bloody well expose it saner ways.  Assuming anyone does debugging
of amiserial in the first place, that is.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>