Remove ALLOW_TOOLHANDLE_NONCONSTNESS definition
ALLOW_TOOLHANDLE_NONCONSTNESS
is no longer used in Gaudi and our cmake
configuration warns about this variable not being used anywhere:
http://atlas-computing.web.cern.ch/atlas-computing/links/distDirectory/gitwww/GITWebArea/nightlies/master/2020-01-12T2132/Athena/x86_64-centos7-gcc8-opt/AthenaExternals/cmake_config.log.html
Edited by Frank Winklmeier