I believe the question was asked and posted here a long time ago, but for a different "Delta Airlines" sign on an old abandoned building easily visible going to or from the new airport.

The answer I recall is that this is a remnant of an old, like over twenty years ago, of failed DL expansion in the far east. Of course, in more recent years, DL did not fly to BKK at all (until it bought NW).

For the second sign, DL would do well to pay to have it painted over.