Windows: apply the workaround for +<basename> removal to +objective subfolder

Because MATLAB under Windows maintains a lock on the “+<basename>” subdirectory,
we use a workaround consisting in first renaming the directory, then deleting
it.

But this is not enough when the “ramsey_policy” command is used, because the
latter creates a “+objective” subfolder inside “+<basename>”, on which MATLAB
also maintains a lock.

The fix consists in recursively using the same workaround as for the top-level
directory: renaming before deleting.