[Style KCM] Don't explicitly reload KWin's configuration

This was only done for changes to the "effects", not the actual widget style, but:

• plasma-integration updates ShowIconsOnPushButtons and ShowIconsInMenuItems when a change to SETTINGS_STYLE is notified by the KCM
• plasma-integration updates toolbar style when ToolbarStyleChanged is notified by the KCM

Differential Revision: https://phabricator.kde.org/D24260