errors: replace .split() with .replace()

Replace a somewhat idiosyncratic use of `split()` to remove a prefix
with `replace()`. (A case could be made for `slice()` as well but I
think this is more readable.)

PR-URL: #15545
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: Trott

lib/internal/errors.js

@@ -311,7 +311,7 @@ function invalidArgType(name, expected, actual) {
   let determiner;
   if (typeof expected === 'string' && expected.startsWith('not ')) {
     determiner = 'must not be';
-    expected = expected.split('not ')[1];
+    expected = expected.replace(/^not /, '');
   } else {
     determiner = 'must be';
   }