Kill RAW_ literals

Syntactically, they are indistinguishable from non-raw versions, so it
doesn't make sense to separate then *at the syntax* level.

crates/syntax/src/ast/node_ext.rs

@@ -55,13 +55,7 @@ impl ast::Attr {
         let key = self.simple_name()?;
         let value_token = lit.syntax().first_token()?;
 
-        let value: SmolStr = if let Some(s) = ast::String::cast(value_token.clone()) {
-            s.value()?.into()
-        } else if let Some(s) = ast::RawString::cast(value_token) {
-            s.value()?.into()
-        } else {
-            return None;
-        };
+        let value: SmolStr = ast::String::cast(value_token.clone())?.value()?.into();
 
         Some((key, value))
     }