Remove unused color crate (#12860)

This PR removes the `color` crate, as it was not used anywhere.

We had added this experimentally, but right now its existence is just a
source of confusion.

Release Notes:

- N/A

crates/theme/Cargo.toml

@@ -20,7 +20,6 @@ doctest = false
 [dependencies]
 anyhow.workspace = true
 collections.workspace = true
-color.workspace = true
 derive_more.workspace = true
 fs.workspace = true
 futures.workspace = true