tests: drop testing trampolim backend

trampolim project looks unmaintained:
https://github.com/FFY00/trampolim

Its build from source tree fails on Python 3.13:
```
Captured stderr:

Traceback (most recent call last):
  File "/tmp/pytest-of-runner/pytest-0/test_build_and_install_in_tree0/venv/lib/python3.13/site-packages/pyproject_installer/lib/backend_helper/backend_caller.py", line 200, in <module>
    main(sys.argv[1:])
    ~~~~^^^^^^^^^^^^^^
  File "/tmp/pytest-of-runner/pytest-0/test_build_and_install_in_tree0/venv/lib/python3.13/site-packages/pyproject_installer/lib/backend_helper/backend_caller.py", line 187, in main
    hook_result = call_hook(
        backend,
    ...<3 lines>...
        hook_kwargs=hook_kwargs,
    )
  File "/tmp/pytest-of-runner/pytest-0/test_build_and_install_in_tree0/venv/lib/python3.13/site-packages/pyproject_installer/lib/backend_helper/backend_caller.py", line 155, in call_hook
    return hook_func(*hook_args, **hook_kwargs)
  File "/tmp/pytest-of-runner/pytest-0/test_build_and_install_in_tree0/trampolim/trampolim/__init__.py", line 59, in build_wheel
    project = trampolim._build.Project()
  File "/tmp/pytest-of-runner/pytest-0/test_build_and_install_in_tree0/trampolim/trampolim/_build.py", line 148, in __init__
    itertools.chain(self.distribution_source, self.build_system_source),
                    ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/pytest-of-runner/pytest-0/test_build_and_install_in_tree0/trampolim/trampolim/_build.py", line 228, in distribution_source
    | self.python_source.source
      ^^^^^^^^^^^^^^^^^^
  File "/tmp/pytest-of-runner/pytest-0/test_build_and_install_in_tree0/trampolim/trampolim/_build.py", line 280, in python_source
    for path in directory.iterdir()
                ~~~~~~~~~~~~~~~~~^^
  File "/opt/hostedtoolcache/Python/3.13.0-rc.2/x64/lib/python3.13/pathlib/_local.py", line 575, in iterdir
    with os.scandir(root_dir) as scandir_it:
         ~~~~~~~~~~^^^^^^^^^^
NotADirectoryError: [Errno 20] Not a directory: 'trampolim/_build.py'

=========================== short test summary info ============================
FAILED tests/integration/test_backends.py::test_build_and_install_in_tree_backends[trampolim] - subprocess.CalledProcessError: Command '['/tmp/pytest-of-runner/pytest-0/test_build_and_install_in_tree0/venv/bin/python', '-m', 'pyproject_installer', 'build']' returned non-zero exit status 1.
```

Fixes: #78
Signed-off-by: Stanislav Levin <[email protected]>

tests/integration/test_backends.py

@@ -70,15 +70,6 @@ def normalized_name(request):
             "hatchling",
             id="hatchling.ouroboros",
         ),
-        pytest.param(
-            (
-                "trampolim",
-                "https://github.com/FFY00/trampolim",
-                None,
-            ),
-            "trampolim",
-            id="trampolim",
-        ),
         pytest.param(
             (
                 "pdm-backend",