Auto-remove extensions that no longer exist from `auto_install_extensions`

[injust]

Summary

After upgrading to 0.208.x, Zed shows "Installing ruff extension..." permanently.

Description

Steps to reproduce:

1. Add { "auto_install_extensions": { "ruff": true } } to settings
2. Restart Zed

Expected Behavior: Zed handles the removal of the Ruff extension gracefully (e.g. remove from settings and don't try to install it)
Actual Behavior: Zed keeps trying to install the Ruff extension on startup, but there are no errors

Zed Version and System Specs

Zed: v0.208.4 (Zed)
OS: macOS 15.7.1
Memory: 16 GiB
Architecture: x86_64

[injust]

Also https://zed.dev/extensions/ruff still exists and needs to be removed.

[smitbarmase]

Currently, uninstalling an extension does not remove it from "auto_install_extensions". I think we should do that, though that decision is opinionated.

[injust]

Currently, uninstalling an extension does not remove it from "auto_install_extensions". I think we should do that, though that decision is opinionated.

I think uninstalling it manually shouldn't remove it from the user's settings. But if the extension is removed and no longer exists, then it probably should be auto-removed from auto_install_extensions, since the extension doesn't exist anymore. Otherwise, it shows up in the status bar as permanently installing.

[injust]

The bug portion of this issue was duplicated in #41597 and fixed in #41551.

The remaining feature is to auto-remove such extensions from auto_install_extensions.