Remove legacy themes and licenses (#24589)

This PR removes Zed legacy themes – all themes except `One` and
`Gruvbox`.

These will likely be renamed in the future (to something like `Zed One`,
`Zed Gruvbox` to allow the original authors of those themes to provide
their own official versions.)

You can grab the
[`zed-legacy-themes`](https://github.com/zed-extensions/legacy-themes)
extension if you would like to continue using one of these themes.

## How to install the extension:

- Go to the extension store (`zed: extensions`, cmd+shift+x on macOS)
- Search for the `Zed Legacy Themes` extension and install it.
- Now the themes will be installed (with names like `Zed Legacy:
Andromeda`)

Release Notes:

- A number of themes are no longer installed in Zed by default:
`Andromeda`, `Atelier`, `Rosé Pine`, `Sandcastle`, `Solarized` &
`Summercamp`. If you would like to continue using one of these
extensions: 1. Open `zed: extensions`, 2. Install the
`zed-legacy-themes` extension. 3. Re-select your desired theme.

Co-authored-by: maxdeviant <elliott.codes@gmail.com>