legs_helper: add Delta seatmaps

Thank you rfett2!

See http://www.flyertalk.com/forum/21129087-post2.html

Signed-off-by: diditclear <didyourupgradeclear@gmail.com>
best_seats: remove UA specific seatmaps

Although various airlines are supported now, Delta won't work
with iFrames (X-frame-options: SAMEORIGIN).

Signed-off-by: diditclear <didyourupgradeclear@gmail.com>