tests: stop testing git rebase --preserve-merges

This backend has been deprecated in favor of `git rebase
--rebase-merges`.

In preparation for dropping it, let's remove all the regression tests
that would need it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.github/workflows/main.yml

@@ -198,7 +198,6 @@ jobs:
       shell: bash
       env:
         NO_SVN_TESTS: 1
-        GIT_TEST_SKIP_REBASE_P: 1
       run: ci/run-test-slice.sh ${{matrix.nr}} 10
     - name: ci/print-test-failures.sh
       if: failure()