cleanup sanity_check_paths in numpy & scipy easyblocks

[boegel]

• stop additional import check, which is already done by default for all Python packages (based on software name)
• stop checking for specific lib/python*/site-packages, rely on det_pylibdir() instead to ensure correct path is checked depending on active python command
• • some minor cleanup

Especially not checking for self.pyblidr is important w.r.t. supporting multi-Python installations of (bundles that include) numpy or scipy (cfr. easybuilders/easybuild-easyconfigs#7921 and easybuilders/easybuild-easyconfigs#7922).

[damianam]

lgtm