Remove built-in Rosé Pine theme

[ThatOneCalculator]

For after https://github.com/zed-industries/extensions/pull/630

Release Notes:

• Removes the built-in Rosé Pine theme in favor of the official theme extension

[ThatOneCalculator]

@zed-industries-bot recheck

[maxdeviant]

We'll need to discuss internally what the plan is for the old themes.

One idea that was being considered was pulling the current built-in themes that get replaced into a separate "Zed Classic Themes" extension (or similar) so that anyone who prefers the current themes can use them.

So I don't think we're ready to remove the Rosé Pine themes just yet.

Installing the extension will overwrite the built-in ones, so it shouldn't prevent anyone from using the official themes.

[maxdeviant]

Going to close this until we decide what the plans are for the built-in themes.