perlin: remove unused import and fix import ordering

[brendancol]

What

Style sweep cleanup for xrspatial/perlin.py. Two findings from the
project's own tooling (setup.cfg), neither gated in CI:

• flake8 F401: not_implemented_func was imported from xrspatial.utils
  and never used. It's a refactor leftover. The name isn't re-exported
  (only perlin leaves the module via xrspatial/__init__.py), so
  dropping it doesn't touch the public API.
• isort: the from xrspatial.utils import (...) line now wraps to the
  configured line_length=100.

Backends

No behavioural change. Style fixes are static and apply uniformly across
numpy / cupy / dask+numpy / dask+cupy.

Test plan

• flake8 xrspatial/perlin.py clean
• isort --check-only --diff xrspatial/perlin.py clean
• import xrspatial.perlin and from xrspatial import perlin both ok
• pytest xrspatial/tests/test_perlin.py -> 12 passed (CUDA available)

Note: GitHub issues are disabled on this repo, so there's no linked
issue. State recorded in .claude/sweep-style-state.csv.

[brendancol]

PR Review: perlin: remove unused import and fix import ordering

Import-only style cleanup. I read perlin.py in full and confirmed the
four names kept on the import line are all referenced in the module:
ArrayTypeFunctionMapping and cuda_args in _perlin_cupy / dispatch,
_dask_task_name_kwargs in both dask paths, _validate_raster in perlin().
not_implemented_func had no use anywhere and isn't re-exported, so
dropping it is safe.

Blockers

None.

Suggestions

None.

Nits

None.

What looks good

• flake8 and isort both clean after the change.
• No behavioural change: the removed name was never called, and the
  re-wrap is whitespace only.
• test_perlin.py passes (12 tests), so the numpy/cupy/dask dispatch
  still imports and runs.

Checklist

• No behavioural change (import-only)
• Remaining imports all used
• Removed name not re-exported (only perlin leaves the module)
• flake8 + isort clean
• Existing tests pass
• [n/a] Algorithm / NaN / dask-boundary / benchmark / README -- no logic touched