Drop XRandR dependency from Options's currentRefreshRate

Summary:
This code branch was dead. It was for the case that the number of screens
is 0, but KWin::Screens never has a count of zero, especially not the
XRandR based screens (c.f. screens_xrandr.cpp method ::update) which has
a fallback handling setting the count to always 1 if there are no screens.

Test Plan: Compiles

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D7650