[effects] Save zoom settings using kcfg generated class

Summary:
We have the kcfg generated settings object, so no need to manually write
the config save code for zoom and magnifier effect.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3338

effects/magnifier/magnifier.cpp

@@ -77,9 +77,8 @@ MagnifierEffect::~MagnifierEffect()
     delete m_texture;
     destroyPixmap();
     // Save the zoom value.
-    KConfigGroup conf = EffectsHandler::effectConfig(QStringLiteral("Magnifier"));
-    conf.writeEntry("InitialZoom", target_zoom);
-    conf.sync();
+    MagnifierConfig::setInitialZoom(target_zoom);
+    MagnifierConfig::self()->save();
 }
 
 void MagnifierEffect::destroyPixmap()

effects/zoom/zoom.cpp

@@ -142,9 +142,8 @@ ZoomEffect::~ZoomEffect()
     // switch off and free resources
     showCursor();
     // Save the zoom value.
-    KConfigGroup conf = EffectsHandler::effectConfig(QStringLiteral("Zoom"));
-    conf.writeEntry("InitialZoom", target_zoom);
-    conf.sync();
+    ZoomConfig::setInitialZoom(target_zoom);
+    ZoomConfig::self()->save();
 }
 
 void ZoomEffect::showCursor()