fdo#68876: sw: layout should never care about IsFirstShared()

With 75084f6c42c27dc95418df9cefed2fddfb26000e the layout should put the
"first" frame-format on every first-page of a page style; unfortunately
SwFrm::OnFirstPage() was not adapted with this change and that results
in various misbehaviour in the layout.

Change-Id: Ia31add0d72ca581963b7e8e40c3ea932361b7fd6
diff --git a/sw/source/core/layout/trvlfrm.cxx b/sw/source/core/layout/trvlfrm.cxx
index afcbe73..ff79507 100644
--- a/sw/source/core/layout/trvlfrm.cxx
+++ b/sw/source/core/layout/trvlfrm.cxx
@@ -1782,7 +1782,7 @@ bool SwFrm::OnFirstPage() const
        if (pPrevFrm)
        {
            const SwPageDesc* pDesc = pPage->GetPageDesc();
            bRet = pPrevFrm->GetPageDesc() != pDesc && !pDesc->IsFirstShared();
            bRet = pPrevFrm->GetPageDesc() != pDesc;
        }
        else
            bRet = true;