Revert "Use header colors for custom toolbar on Main page"

This reverts commit af2c76ee.

It's unnecessary now that ToolBar colors come from the QQC2 style in
the manner that we need here.

src/data/contents/ui/MainPage.qml

@@ -182,8 +182,6 @@ Kirigami.ScrollablePage {
     }
     header: QQC2.ToolBar {
         id: toolbar
-        Kirigami.Theme.colorSet: Kirigami.Theme.Header
-        Kirigami.Theme.inherit: false
         RowLayout {
             anchors.fill: parent
             Kirigami.SearchField {