Remove the GetStandardProperties trait.

This was using dynamic dispatch to deal with enums to avoid the repetitive typing.

src/parser/plain_text.rs

@@ -143,7 +143,7 @@ mod tests {
     use crate::context::GlobalSettings;
     use crate::context::List;
     use crate::parser::object_parser::detect_standard_set_object_sans_plain_text;
-    use crate::types::GetStandardProperties;
+    use crate::types::StandardProperties;
 
     #[test]
     fn plain_text_simple() {
@@ -160,9 +160,6 @@ mod tests {
         )(input)
         .unwrap();
         assert_eq!(Into::<&str>::into(remaining), "");
-        assert_eq!(
-            result.get_standard_properties().get_source(),
-            Into::<&str>::into(input)
-        );
+        assert_eq!(result.get_source(), Into::<&str>::into(input));
     }
 }