edit prediction: Don't show and discard completion if toggled off in the buffer (#24927)

Discards an inline completion when it's toggled to off (using, say, a
keyboard shortcut). This matches the behaviour in VS Code and JetBrains,
and I think is a bit more intuitive.

(https://github.com/zed-industries/zed/discussions/24895)

Release Notes:

- N/A