Remove handler dict ADTs from Lowering
Remove handler dict ADTs from Lowering

They were removed in #1072, which was merged into `main` after the last
CI run in #1074.

google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang