Fix realDpi function for Mac

Summary:
Removes old Utils::realDpi function for Mac. The old code employs functions
CGDisplayCurrentMode and CGDisplayIOServicePort, which are deprecated as
of versions 10.6 and 10.9, respectively. Now Mac uses the same code, as
Linux does, since it works out of the box.

This commit automatically fixes the long-standing bug of having an
implementation of realDpiX() and realDpiY(), which are not declared in
the utils.h file.

Reviewers: #okular, rjvbb

Reviewed By: rjvbb

Subscribers: rjvbb, cullmann, aacid, kde-mac, ltoscano, #okular

Tags: #okular

Differential Revision: https://phabricator.kde.org/D10415