Revert "[doc] Mention improvements of MatrixIndexSet in changelog"

This reverts commit 107a54a8.

CHANGELOG.md

@@ -16,11 +16,6 @@ SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception
   However, internally the `BCCSMatrixInitializer` is replaced by direct calls of `flatMatrixForEach` similar to `Cholmod`. This requires a
   compatible vector field of "ignored" degrees of freedom. The method `setSubMatrix` with a top-level index set is preserved for compatibility.
 
-- The internal storage in `MatrixIndexSet` was changed from `std::set` to a sorted `std::vector`
-  (with a `std::set` fallback for very dense rows) to improve performance. The stored index
-  type was changed from `std::size_t` to `uint32_t` to reduce memory consumption and improve
-  performance. Hence, `MatrixIndexSet` can no longer be used for very large matrices with more
-  than 2^32 columns.
 
 - Added flag 'useFixedOrder' to the coarsen method of AMGs ParallelIndicesCoarsener.
   If set to true, during the creation of the coarser matrix (by accumulation and restriction