Remove the GetStandardProperties trait.

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

src/parser/paragraph.rs

@@ -96,7 +96,7 @@ mod tests {
     use crate::context::List;
     use crate::parser::element_parser::element;
     use crate::parser::org_source::OrgSource;
-    use crate::types::GetStandardProperties;
+    use crate::types::StandardProperties;
 
     #[test]
     fn two_paragraphs() {
@@ -109,13 +109,7 @@ mod tests {
         let (remaining, second_paragraph) =
             paragraph_matcher(remaining).expect("Parse second paragraph.");
         assert_eq!(Into::<&str>::into(remaining), "");
-        assert_eq!(
-            first_paragraph.get_standard_properties().get_source(),
-            "foo bar baz\n\n"
-        );
-        assert_eq!(
-            second_paragraph.get_standard_properties().get_source(),
-            "lorem ipsum"
-        );
+        assert_eq!(first_paragraph.get_source(), "foo bar baz\n\n");
+        assert_eq!(second_paragraph.get_source(), "lorem ipsum");
     }
 }