assert: remove code that is never reached

The internal function `truncate()` is only called with the first
argument being the output of `util.inspect()`. `util.inspect()` calls
its own internal `formatValue()` which is guaranteed to return a string.

Therefore, we can remove the check in `truncate()` that the first
argument is a string as well as code to handle the case where it is not
a string.

PR-URL: #8132
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Alexander Makarenko <estliberitas@gmail.com>
Reviewed-By: Yorkie Liu <yorkiefixer@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/assert.js

@@ -61,11 +61,7 @@ assert.AssertionError = function AssertionError(options) {
 util.inherits(assert.AssertionError, Error);
 
 function truncate(s, n) {
-  if (typeof s === 'string') {
-    return s.length < n ? s : s.slice(0, n);
-  } else {
-    return s;
-  }
+  return s.slice(0, n);
 }
 
 function getMessage(self) {