Disable the span feature temporarily

It's broken on nightly and needs fixing

crates/backend/Cargo.toml

@@ -11,7 +11,7 @@ Backend code generation of the wasm-bindgen tool
 """
 
 [features]
-spans = ["proc-macro2/nightly"]
+spans = []
 
 [dependencies]
 quote = '0.5'

crates/macro/Cargo.toml

@@ -14,7 +14,7 @@ Definition of the `#[wasm_bindgen]` attribute, an internal dependency
 proc-macro = true
 
 [features]
-spans = ["proc-macro2/nightly", "wasm-bindgen-backend/spans"]
+spans = ["wasm-bindgen-backend/spans"]
 
 [dependencies]
 syn = { version = '0.13', features = ['full'] }