Remove unused function

We do not offer explicit diary functionality.

Either a user creates a diary implicitely by creating a new entry inside
of it, or not.

imag-diary/src/main.rs

@@ -57,7 +57,6 @@ fn main() {
                 "delete" => delete(&rt),
                 "edit" => edit(&rt),
                 "list" => list(&rt),
-                "diary" => diary(&rt),
                 "view" => view(&rt),
                 _        => {
                     debug!("Unknown command"); // More error handling
@@ -66,7 +65,3 @@ fn main() {
         });
 }
 
-fn diary(rt: &Runtime) {
-    unimplemented!()
-}
-