gpui: Remove dependency on sqlez (#3871)

This removes one of the path dependencies in gpui that's only really
needed by `workspace` (which can work around lack of these
implementations by itself). In theory it should also improve build
scheduling (as gpui doesn't have to wait for main dependency of sqlez -
libsqlite3 - to finish it's 25 seconds-long build in release), though in
practice I didn't notice a substantial improvement.
Moreover `sqlez` was unused by `settings` too, so that's removed
as well.

Release Notes:

- N/A