tsm: screen: remove unused get_from_scrollback()

tsm: screen: remove unused get_from_scrollback()

This function is not used and I do not intend to use it anywhere soon.
Therefore, remove it so it doesn't generate sparse warnings.

The only reason why I kept is was that we might want to retrieve these
when increasing window size. However, this will not be implemented in the
near future as we need to keep a buffer-fill state for it.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>