This commit works around an issue with the "clean"
command (see [1]) that was not properly considering the current
MACOSX_DEPLOYMENT_TARGET env. variable.

It does by explicitly deleting the scikit-build directory
without relying on the "setup.py clean" command not respecting the
selected deployment target.

Considering that

1. the libraries installed while building the dependent itk-* wheels
   were accumulated in the install directory (e.g `_skbuild/macosx-X.Y-<arch>-3.9/setuptools/lib`)

2. the wheel is generated by systematically archiving the content of
   the install directory. The manifest files are used to select which
   file to "copy" into the install directory but these are not used
   by the function creating the wheel archive. See [2]

3. following scikit-build/scikit-build@ac9648bf4 (post 0.8.1), the
   platform name was properly considered on macOS *only* when
   building the wheel (using "setup.py build") but not when cleaning
   the build directory (using "setup.py clean")

... all the wheels depending on "itk-core" where unexpectedly large because
there are including the cumulative set of files from all the wheels built
beforehand.

[1] scikit-build/scikit-build#1012
[2] https://github.com/pypa/wheel/blob/0.40.0/src/wheel/wheelfile.py#L121-L141

Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>
Co-authored-by: Matt McCormick <matt.mccormick@kitware.com>