fix: remove empty class attribute left behind after settle/swap/request (#3715)

fix: remove empty class attribute left behind after htmx operations

After htmx removes its utility classes (swapping, settling, request),
elements are left with an empty class="" attribute if they had no other
classes. This happens because doSettle and the request indicator cleanup
use raw classList.remove instead of removeClassFromElement, which
already handles empty class attribute removal.

Replace classList.remove with removeClassFromElement at three sites:
- swappingClass removal (line 1978)
- settlingClass removal in doSettle (line 1999)
- requestClass removal from indicators (line 3415)

Fixes #1701
Supersedes #2683