Unused / undeclared variable in Config.php

Issue #1929136: Fix override-free context, move global config overrides back to an event listener removed the declaration protected $overrides from the Config class, but neglected to remove the initialization of this variable which takes place in the init() function: $this->overrides = array();

Issue #1971158: Follow-up: Add loadMultiple() and listAll() caching to (cached) config storage compounded the error by creating a new function initWithData() which was copied in part from the existing function init() - this copy included the initialization of the removed $overrides variable.

As a result, the Config class now has an undeclared/undocumented/unused public variable called $overrides which doesn't do anything. Attached patch removes this.