fix mypy type check

Address this:
```
src/alchemlyb/preprocessing/subsampling.py:454: error: Unused "type: ignore" comment  [unused-ignore]
pyproject.toml: note: unused section(s): module = ['_pytest.*', 'alchemtest.*', 'pytest.*']
Found 1 error in 1 file (checked 31 source files)
```

