tdf#158773 reduce time spent in IndexedStyleSheets::Reindex

no need to de-allocate and then re-allocate the vectors inside
mStyleSheetPositionsByFamily

Change-Id: I3ad10173d9f3ba252619667afe13250045c943c5
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/164529
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
diff --git a/svl/source/items/IndexedStyleSheets.cxx b/svl/source/items/IndexedStyleSheets.cxx
index 57e2ddd..c4b879d 100644
--- a/svl/source/items/IndexedStyleSheets.cxx
+++ b/svl/source/items/IndexedStyleSheets.cxx
@@ -64,9 +64,9 @@ void
IndexedStyleSheets::Reindex()
{
    mPositionsByName.clear();
    mStyleSheetPositionsByFamily.clear();
    mStyleSheetPositionsByFamily.resize(NUMBER_OF_FAMILIES);
    for (size_t i = 0; i < NUMBER_OF_FAMILIES; i++) {
        mStyleSheetPositionsByFamily.emplace_back();
        mStyleSheetPositionsByFamily[i].clear();
    }

    sal_Int32 i = 0;