Revert "Export missing functionality in MatrixIndexSet"

This reverts commit 3d23f92a.

dune/istl/matrixindexset.hh

@@ -142,22 +142,6 @@ namespace Dune {
     /** \brief Return the number of rows */
     size_type rows() const {return rows_;}
 
-    /** \brief Return the number of columns */
-    size_type cols() const {return cols_;}
-
-    /**
-     * \brief Return column indices of entries in given row
-     *
-     * This returns a range of all column indices
-     * that have been added for the given column.
-     * Since there are different internal implementations
-     * of this range, the result is stored in a std::variant<...>
-     * which has to be accessed using `std::visit`.
-     */
-    const auto& columnIndices(size_type row) const {
-      return indices_[row];
-    }
-
     /** \brief Return the number of entries in a given row */
     size_type rowsize(size_type row) const {
       return std::visit([&](const auto& rowIndices) {