chore: remove unused dependencies discovered by cargo-machete

nvim-core/Cargo.toml

@@ -6,7 +6,6 @@ edition = "2021"
 [dependencies]
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
-rmp-serde = "1.1"
 rmpv = "1.0"
 tokio = { version = "1.37", features = ["full", "io-util", "io-std"] }
 tracing = "0.1.44"