R1:d9592e034812

Removed unused validateNavigationMenuItem() from PhabricatorProfileMenuEngine

Summary:
The commit rP7bde92b9c9806c5967df617fb4e843dea315ecf7 (2016) introduced `private function validateNavigationMenuItem()`.
The only call to this method was removed in rP950e9d085b482bc3484e2066591ac07c20154c9a.
Thus remove this unused method.

Test Plan:
Read/grep the code in `PhabricatorProfileMenuEngine`.

Run also `grep -RF 'validateNavigationMenuItem' --include="*.php" .` to verify that this is an orphan method.