tty: vt, drop VT_BUF_VRAM_ONLY

It is never defined. And I spent quite some time looking into the
history and cannot find how this was ever used. Given it was not used
in the history, I doubt it currently works as expected after the years
of changes all over the code.

So kill it.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>