Fix -Wredundant-move

Summary:
    /home/vlad/Workspace/KDE/src/kde/workspace/kwin/kcmkwin/kwindecoration/declarative-plugin/previewbridge.cpp: In member function ‘virtual std::unique_ptr<KDecoration2::DecoratedClientPrivate> KDecoration2::Preview::PreviewBridge::createClient(KDecoration2::DecoratedClient*, KDecoration2::Decoration*)’:
    /home/vlad/Workspace/KDE/src/kde/workspace/kwin/kcmkwin/kwindecoration/declarative-plugin/previewbridge.cpp:63:21: warning: redundant move in return statement [-Wredundant-move]
       63 |     return std::move(ptr);
          |            ~~~~~~~~~^~~~~
    /home/vlad/Workspace/KDE/src/kde/workspace/kwin/kcmkwin/kwindecoration/declarative-plugin/previewbridge.cpp:63:21: note: remove ‘std::move’ call
    /home/vlad/Workspace/KDE/src/kde/workspace/kwin/kcmkwin/kwindecoration/declarative-plugin/previewbridge.cpp: In member function ‘virtual std::unique_ptr<KDecoration2::DecorationSettingsPrivate> KDecoration2::Preview::PreviewBridge::settings(KDecoration2::DecorationSettings*)’:
    /home/vlad/Workspace/KDE/src/kde/workspace/kwin/kcmkwin/kwindecoration/declarative-plugin/previewbridge.cpp:81:21: warning: redundant move in return statement [-Wredundant-move]
       81 |     return std::move(ptr);
          |            ~~~~~~~~~^~~~~

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D22357