At the same time, since the new tag uses LCG_97apython3_ATLAS_1 by default
for all LCG based projects, removed the overrides from the build_externals.sh
scripts of all projects for forcing the usage of LCG_97a_ATLAS_1.

Also updated the Athena configuration to use the TDAQ installations compatible
with Python 3 instead of Python 2.7, and removed the NumPy environment setup
from the project. Since with LCG_97a we don't need that anymore.