Use niworkflows enhance-and-skullstrip workflow

[tsalo]

Closes #353.

Changes proposed in this pull request

• Remove aslprep/workflows/asl/util.py.
• Replace aslprep.workflows.asl.util.init_enhance_and_skullstrip_asl_wf with niworkflows.func.util.init_enhance_and_skullstrip_bold_wf.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (0fc3ec6) 79.02% compared to head (24f4d6e) 78.89%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #371 +/- ## ========================================== - Coverage 79.02% 78.89% -0.14% ========================================== Files 39 38 -1 Lines 4157 4131 -26 Branches 602 602 ========================================== - Hits 3285 3259 -26 Misses 676 676 Partials 196 196 ```

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