Dont treat python and numpy as regular external projects since they are not built. Remove from external projects list.

Summary: see above

Test Plan: dartclients should run with enabled python now

Reviewers: O1 MITK Core, kislinsk

Reviewed By: O1 MITK Core, kislinsk

Maniphest Tasks: T25255

Differential Revision: https://phabricator.mitk.org/D157