menuconfig: Do not open code textbox scroll up/down

We don't need to explicitely use ncurses' scroll(). ncurses performs
vertical-motion optimization at wrefresh() time.

Using strace I confirmed that with the following patch curses still sends only
the new line of text to the terminal when scrolling up/down one line at a
time.

Signed-off-by: Benjamin Poirier <bpoirier@suse.de>
Signed-off-by: Michal Marek <mmarek@suse.cz>