Alias santa_bra -> santa_bikini

BUR #61755 is pending approval.

create alias santa_bra -> santa_bikini

While adding images to the bra wiki on Danbooru, I noticed that Santa bra and Santa bikini are extremely difficult to distinguish from each other. I believe there is no need to list Santa bra as a separate tag.

While quite a few could be moved, there are posts like post #10850393 and post #5957501 that are clearly bras and not bikinis.

Somewhat relevant: topic #29901 (cleanup of our santa_* tags)