update: duration_format renders deprecation warning

[BeyondEvil]

Remove the duration_formatter and warn if it's used as it's no longer functional in 4.x.

[BeyondEvil]

Why is the test failing?

From the output the deprecation warning is being emitted, so why is it failing?

https://github.com/pytest-dev/pytest-html/actions/runs/4457713820/jobs/7828947129?pr=589#step:10:335

Stacktrace

testing/test_unit.py ============================= test session starts ==============================
platform linux -- Python 3.10.10, pytest-7.2.2, pluggy-1.0.0
rootdir: /tmp/pytest-of-runner/pytest-0/test_duration_format_deprecation_warning0
plugins: html-4.0.0rc0, metadata-2.0.4, cov-4.0.0, xdist-3.2.1, mock-3.10.0, rerunfailures-11.1.2
collected 1 item
test_duration_format_deprecation_warning.py F
=================================== FAILURES ===================================
__________________________________ test_pass ___________________________________
def test_pass():
with pytest.warns(DeprecationWarning) as record:

      assert len(record) == 1

E assert 0 == 1
E + where 0 = len(WarningsChecker(record=True))
test_duration_format_deprecation_warning.py:4: AssertionError
=============================== warnings summary ===============================
test_duration_format_deprecation_warning.py::test_pass
test_duration_format_deprecation_warning.py::test_pass
test_duration_format_deprecation_warning.py::test_pass
/home/runner/work/pytest-html/pytest-html/.tox/py310-cov/lib/python3.10/site-packages/pytest_html/nextgen.py:244: DeprecationWarning: 'duration_formatter' has been removed and no longer has any effect!
warnings.warn(
-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

• Generated html report: file:///tmp/pytest-of-runner/pytest-0/test_duration_format_deprecation_warning0/report.html -
  =========================== short test summary info ============================
  FAILED test_duration_format_deprecation_warning.py::test_pass - assert 0 == 1
  ======================== 1 failed, 3 warnings in 0.02s =========================
  F
  =================================== FAILURES ===================================
  ___________________ test_duration_format_deprecation_warning ___________________
  pytester = <Pytester PosixPath('/tmp/pytest-of-runner/pytest-0/test_duration_format_deprecation_warning0')>
  def test_duration_format_deprecation_warning(pytester):
  pytester.makeconftest(
  """
  import pytest
  @pytest.hookimpl(hookwrapper=True)
  def pytest_runtest_makereport(item, call):
  outcome = yield
  report = outcome.get_result()
  setattr(report, "duration_formatter", "%H:%M:%S.%f")
  """
  )
  pytester.makepyfile(
  """
  import pytest
  def test_pass():
  with pytest.warns(DeprecationWarning) as record:
  assert len(record) == 1
  """
  )
  result = run(pytester)

  assert result.ret == 0

E assert <ExitCode.TESTS_FAILED: 1> == 0
E + where <ExitCode.TESTS_FAILED: 1> = <RunResult ret=ExitCode.TESTS_FAILED len(stdout.lines)=30 len(stderr.lines)=0 duration=0.12s>.ret
/home/runner/work/pytest-html/pytest-html/testing/test_unit.py:29: AssertionError