Delete .cache dir on wipe

[stsewd]

We are not deleting this directory, here is where PIP caches its
packages.

readthedocs.org/readthedocs/projects/models.py

Lines 673 to 677 in c3001be

	@property
	def pip_cache_path(self):
	"""Path to pip cache."""
	return os.path.join(self.doc_path, '.cache', 'pip')

Users can still get the old (buggy) PIP version because of this.

But why wiping worked for a lot of people?

Because after wiping, we recreate the virtual environment and looks like
PIP doesn't use the packages from the cache.
But in the next build, when reusing the virtual environment, it will use
the packages from the cache.

ref #6554 (comment)

[stsewd]

May fix #5810

[stsewd]

Also, this is why we didn't remove it before #5685

[Aricg]

Thanks guys!