Remove unused Display implementations

After the previous commit, nushell uses PrettyDebug and
PrettyDebugWithSource for our pretty-printed display output.

PrettyDebug produces a structured `pretty.rs` document rather than
writing directly into a fmt::Formatter, and types that implement
`PrettyDebug` have a convenience `display` method that produces a string
(to be used in situations where `Display` is needed for compatibility
with other traits, or where simple rendering is appropriate).

tests/helpers/mod.rs

@@ -6,6 +6,7 @@ pub use std::path::PathBuf;
 
 use app_dirs::{get_app_root, AppDataType};
 use getset::Getters;
+use nu_source::PrettyDebug;
 use std::io::Read;
 use tempfile::{tempdir, TempDir};
 
@@ -45,7 +46,7 @@ impl DisplayPath for &String {
 
 impl DisplayPath for nu::AbsolutePath {
     fn display_path(&self) -> String {
-        self.to_string()
+        self.display()
     }
 }