[#801] Drop the rerunFailingTestsCount setting that never applied to TestNG

[vharseko]

Follow-up of #792 (item 3 of its Proposed fixes).

rerunFailingTestsCount=3 was configured for the precommit profile
(opendj-server-legacy/pom.xml:1284), but it never rerun anything: these tests are executed by
the surefire TestNG provider, which does not implement the option. Up to and including
3.5.x it is honoured by the JUnit 4/5 providers only, and
SUREFIRE-2134 ("add support for
rerunFailingTestsCount option to TestNG provider", open since December 2022) is still
unresolved.

Contrary to the suspicion in #792, this is not a @DataProvider specific limitation - the
option is a no-op for every one of the 31760 tests of the suite.

How it was verified

A minimal project with TestNG 6.14.3, rerunFailingTestsCount=3 and the same
reuseForks=false / forkCount=1 / parallel=none settings as the precommit profile, with
two tests that fail on their first attempt only - a plain @Test and a @DataProvider driven
one:

• surefire 3.5.3 (our version) and 3.5.6 (latest stable):
  Using auto detected provider org.apache.maven.surefire.testng.TestNGProvider,
  Tests run: 6, Failures: 2 -> BUILD FAILURE. No reruns, no Flakes: section, no rerun
  elements in TEST-*.xml.
• Cross-checks: the surefire-testng-3.5.3.jar classes never read
  TestRequest.getRerunFailingTestsCount(), and AbstractSurefireMojo.warnIfRerunClashes()
  only validates that the value is not negative - nothing warns that the provider ignores it,
  which is why the setting looked functional. The rerun-failing-tests page of the 3.5.3 docs
  says: "This feature is supported for JUnit 4.x, and (since 3.0.0-M4) JUnit 5.x."
• surefire 3.6.0-M1 for comparison:
  Using auto detected provider org.apache.maven.surefire.junitplatform.JUnitPlatformProvider
  • TestNG is present. Adding per default org.junit.support:testng-engine:1.1.0, and both
    tests are rerun and reported as Flakes -> BUILD SUCCESS. Our TestNG listener
    configuration (listener / usedefaultlisteners properties) survives that switch, and
    testng-engine supports TestNG from 6.14.3, but a rerun re-executes the whole test method
    (all invocations of its data provider, ~360 for testSafeDataFromRS), and 3.6.0 is currently
    a milestone only. Switching this suite to the unified provider is a migration of its own, not
    part of this change.

Change

Remove the setting and put the reasoning in its place, so that a failing precommit run is
read as a real failure instead of "the retry did not help".

Fixes #801