Fix focus HFR-averaging bug

Summary:
Code intended to remove HFR values that were 2-sigma away from the mean HFR when using the full-field focus algorithm.
However, std::remove_if was not used correctly, and this resulted in a no-op--nothing removed from the HFR mean
calculations. This fixes that issue, and will remove the HFR values > 2-sigma from the HFR mean in the HFR-mean calculation.

Reviewers: mutlaqja

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D24352