Remove old Theme definition (#3195)

This PR removes the old `Theme` definition in favor of the new
`ThemeVariant`s.

The new `SyntaxStyles` have been reverted to the old `SyntaxTheme` that
operates by storing the syntax styles as a vector of
`gpui2::HighlightStyle`s.

This is necessary for the intended usage by `language2`, where we find
the longest key in the theme's syntax styles that matches the capture
name:

https://github.com/zed-industries/zed/blob/18431051d9d750d9e66284a71f7a55a1e31c1374/crates/language2/src/highlight_map.rs#L15-L41

Cargo.lock

@@ -8769,21 +8769,6 @@ dependencies = [
  "util",
 ]
 
-[[package]]
-name = "theme_converter"
-version = "0.1.0"
-dependencies = [
- "anyhow",
- "clap 4.4.4",
- "convert_case 0.6.0",
- "gpui2",
- "log",
- "rust-embed",
- "serde",
- "simplelog",
- "theme2",
-]
-
 [[package]]
 name = "theme_selector"
 version = "0.1.0"