switch to elisa application icon and remove the workaround for icons

Summary:
switch to elisa application icon and remove the workaround for icons

in some KF5 versions, we had icons not playing nice with Elisa so we
added a workaround for them, let's remove it and require a working
version of KF5

use the elisa icon for the application icon

Test Plan: icons look nice

Reviewers: Elisa, paullesur, ngraham

Reviewed By: ngraham

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D12725