According to Screenrant (beware: SPOILERS for future movies):
the originally planned post-credits scene featuring Bruce Banner
a.k.a. Hulk (Mark Ruffalo) would have been shot for the film but
instead, as we learned from co-writer Christopher Markus,
I'll put the information regarding the possible appearance in a future movie in spoilers:
Marvel decided to save the story of where the Hulk is until later (Thor: Ragnarok).
The reason the Hulk-featuring teaser scene didn't make the movie is explained in the linked article:
Christopher Markus, McFeely’s writing partner, explains in the same interview with HuffPo how Ruffalo almost did have a post-credits cameo:
“We can clean that up real quickly. Mark Ruffalo shot no scenes. We talked about having Bruce Banner at the very end of the film, and it just, again, seemed like we’re sticking people in just to stick them in. He has clearly gone somewhere at the end of ‘Ultron,’ and that’s a story. Don’t blow it off and put it in a little tiny chunk just to put in a little extra filigree on our movie. That’s a story.”