Bug 1622256 - Remove deprecated Error::description and use Display trait instead. r=jgraham,webdriver-reviewers

Depends on D66765

Differential Revision: https://phabricator.services.mozilla.com/D66766

--HG--
extra : moz-landing-system : lando

testing/mozbase/rust/mozrunner/src/runner.rs

@@ -89,21 +89,17 @@ pub enum RunnerError {
 
 impl fmt::Display for RunnerError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        self.description().fmt(f)
+        match *self {
+            RunnerError::Io(ref err) => match err.kind() {
+                ErrorKind::NotFound => "no such file or directory".fmt(f),
+                _ => err.fmt(f),
+            }
+            RunnerError::PrefReader(ref err) => err.fmt(f),
+        }
     }
 }
 
 impl Error for RunnerError {
-    fn description(&self) -> &str {
-        match *self {
-            RunnerError::Io(ref err) => match err.kind() {
-                ErrorKind::NotFound => "no such file or directory",
-                _ => err.description(),
-            },
-            RunnerError::PrefReader(ref err) => err.description(),
-        }
-    }
-
     fn cause(&self) -> Option<&dyn Error> {
         Some(match *self {
             RunnerError::Io(ref err) => err as &dyn Error,