Remove elementtree python external, use from Python ditribution

elementtree is now part of standard Python libraries. Boost by
default first attempts to use it from Python.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #269 from davidlt/remove-elementtree

[CMSSW_7_0_X] Remove elementtree python external, use from Python ditribution