In a single zany sentence, this is how the once-promising summer of boxing ended: Triller, a social video app that is a much less popular version of TikTok, put on a pay-per-view fight between a 58-year-old Evander Holyfield (who hasn’t fought in a decade) and a 44-year-old mixed martial artist, Vitor Belfort — and paid former President Donald J. Trump and Donald Trump Jr. to serve as live commentators, all on the 20th anniversary of the Sept. 11 attacks.