Replaced NTA_ROOTDIR by NTA and discarded PYTHONPATH

$NTA_ROOTDIR is redundant. Its remotion decreases the number of environment variables to the user set. 

$PYTHONPATH is not so important to point deserve our concern. It is used in RegionImplFactory.cpp to get $NTA location.. If $NTA always should being set, we don't need transverse $PYTHONPATH to decipher $NTA location.. This is non sense...