[pkg] fix debian packaging (closes #3058542)
- remove useless Python 2.4 reference,
- simplify dh_install configuration to install files consistently instead of
having half in /usr/lib/pythonXX and half in /usr/share/pyshared,
- incidentally, this makes the package work on squeeze.
Regression from eaa58d1c7d5f
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/