Remove useless function

The `columnIdToValueMap()` function was designed to access to the
sparse description of lines of a `SparseMatrixDescriptor`. This was
added when interfacing with `PETSc` but finally the coupling is made
though `CRSMatrix`.

src/algebra/SparseMatrixDescriptor.hpp

@@ -22,12 +22,6 @@ class SparseMatrixDescriptor
     std::map<IndexType, DataType> m_id_value_map;
 
    public:
-    const auto&
-    columnIdToValueMap() const
-    {
-      return m_id_value_map;
-    }
-
     IndexType
     numberOfValues() const noexcept
     {