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.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25719