Revert PR #8327 - Fix autocomplete completions being cut in half (#10084)

This reverts https://github.com/zed-industries/zed/pull/8327

That PR introduced a regression where completions' syntax highlighting
would be corrupted in a non-deterministic way, such that it varied from
frame to frame:

In the screenshot below, many of the field names (e.g. `cursor`,
`depth`) are incorrectly colored as types instead of fields. The
`finished_states` field has highlighting that changes at the wrong
offset. All of these values changed from frame to frame, creating a
strange flickering effect:

<img width="599" alt="Screenshot 2024-04-01 at 5 56 36 PM"
src="https://github.com/zed-industries/zed/assets/326587/b6a48f02-f146-4f76-92e6-32fb417d86c0">

Release Notes:

- N/A