Remove the GetStandardProperties trait.

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

src/parser/citation.rs

@@ -211,7 +211,6 @@ mod tests {
     use crate::context::List;
     use crate::parser::element_parser::element;
     use crate::types::Element;
-    use crate::types::GetStandardProperties;
     use crate::types::StandardProperties;
 
     #[test]
@@ -227,10 +226,7 @@ mod tests {
             _ => panic!("Should be a paragraph!"),
         };
         assert_eq!(Into::<&str>::into(remaining), "");
-        assert_eq!(
-            first_paragraph.get_standard_properties().get_source(),
-            "[cite:@foo]"
-        );
+        assert_eq!(first_paragraph.get_source(), "[cite:@foo]");
         assert_eq!(first_paragraph.children.len(), 1);
 
         match first_paragraph