Remove 'custom file headers' feature.
A file .kdev_python_header could be placed in the root of a project,
the contents of which would be inserted at the top of every file
before parsing.
The suggested use was to avoid python3 syntax errors by adding
from __future__ import print_function, which is no longer needed,
and it's unlikely anyone is actually using this now.
See commit 2e408a96 and
http://blog.svenbrauch.de/2011/12/04/about-python-3-support-in-kdevpython/