Disable syn default features (parcel-bundler#361)

This disables the default syn features in lightningcss-derive's
Cargo.toml.

It also removes an unused syn feature.

Related: parcel-bundler#357

Author: chinedufn

Cargo.lock

@@ -11,6 +11,6 @@ repository = "https://github.com/parcel-bundler/lightningcss"
 proc-macro = true
 
 [dependencies]
-syn = { version = "1.0", features = ["extra-traits"] }
+syn = { version = "1.0", default-features = false }
 quote = "1.0"
 proc-macro2 = "1.0"