installer: Undeprecate WebInstaller::getInfoBox(), still used

getInfoBox() is still widely used in the installer itself and shouldn't have
been hard deprecated in the first place. There's also no proposed replacement
documented, unlike the other *Box functions that were deprecated at the
same time.

This follows up d6ed082b6d66.

Bug: T232903
Bug: T257997
Change-Id: I2ea8d1fd49a87ce4eb0382744312c7ffaefa5f81