gpui: Remove unsound await_on_background helper (#56132)

The function is unsound due to the classic fact that one can leak tasks,
sidestepping the blocking drop behavior resulting in a use after free.

Release Notes:

- N/A or Added/Fixed/Improved ...
c8f0026867Lukas Wirth committed on 5/8/2026, 9:30:34 AM· committed by GitHubparente727080
17 files changedLine totals unavailable