dev-python/pip can probably be removed

[bartoldeman]

This was added because of Reframe being installed seperately.
EESSI/compatibility-layer#72
However since sys-cluster/reframe was added later there is no more need for pip.

Removing pip from the compat layer is a great push to get users to load a Python module instead of pip installing things with the compat layer python.

[boegel]

With the changes in easybuilders/easybuild-easyblocks#2805, EasyBuild v4.6.2 and newer will prefer using pip install (over setup.py install) to install EasyBuild, so keeping pip in the compat layer probably makes sense.
Without it, we would need to use setup.py install instead, and that's a deprecated installation mechanism in recent versions of setuptools (and it actually seems broken too)