Revert "Merge branch 'ExternalsAndBuildUpdate-master-20191202' into 'master'"

This reverts merge request !28428 (merged)

Until I figure out exactly what's going wrong, let's revert this. As it hurts all of the nightlies right now. 😦

For a bit more detail... I see the following in the build logs on the nightly machine, of how it tried to handle the atlas_export_sanitizer.cmake.in files:

...
-- Installing: /build/atnight/localbuilds/nightlies/master/build/install/AthenaExternals/22.0.9/InstallArea/x86_64-centos7-gcc8-opt/cmake/modules/skeletons/SerCnv_entries.cxx.in
-- Installing: /build/atnight/localbuilds/nightlies/master/build/install/AthenaExternals/22.0.9/InstallArea/x86_64-centos7-gcc8-opt/cmake/modules/skeletons/atlas_export_sanitizer.cmake.in
-- Installing: /build/atnight/localbuilds/nightlies/master/build/install/AthenaExternals/22.0.9/InstallArea/x86_64-centos7-gcc8-opt/./packages.txt
-- Installing: /build/atnight/localbuilds/nightlies/master/build/install/AthenaExternals/22.0.9/InstallArea/x86_64-centos7-gcc8-opt/./compilers.txt
-- Installing: /build/atnight/localbuilds/nightlies/master/build/install/AthenaExternals/22.0.9/InstallArea/x86_64-centos7-gcc8-opt/cmake/AthenaExternalsConfig-targets.cmake
-- Installing: /build/atnight/localbuilds/nightlies/master/build/install/AthenaExternals/22.0.9/InstallArea/x86_64-centos7-gcc8-opt/cmake/AthenaExternalsConfig-targets-relwithdebinfo.cmake
-- Installing: /build/atnight/localbuilds/nightlies/master/build/install/AthenaExternals/22.0.9/InstallArea/x86_64-centos7-gcc8-opt/cmake/AthenaExternalsConfig.cmake
-- Installing: /build/atnight/localbuilds/nightlies/master/build/install/AthenaExternals/22.0.9/InstallArea/x86_64-centos7-gcc8-opt/cmake/AthenaExternalsConfig-version.cmake
-- Installing: /build/atnight/localbuilds/nightlies/master/build/install/AthenaExternals/22.0.9/InstallArea/x86_64-centos7-gcc8-opt/./setup.sh
-- Installing: /build/atnight/localbuilds/nightlies/master/build/install/AthenaExternals/22.0.9/InstallArea/x86_64-centos7-gcc8-opt/./ReleaseData
-- Installing: /build/atnight/localbuilds/nightlies/master/build/install/AthenaExternals/22.0.9/InstallArea/x86_64-centos7-gcc8-opt/cmake/PostConfig.cmake
-- Up-to-date: /build/atnight/localbuilds/nightlies/master/build/install/AthenaExternals/22.0.9/InstallArea/x86_64-centos7-gcc8-opt/cmake/modules/skeletons/atlas_export_sanitizer.cmake.in
-- Installing: /build/atnight/localbuilds/nightlies/master/build/install/AthenaExternals/22.0.9/InstallArea/x86_64-centos7-gcc8-opt/cmake/modules/GaudiCPackSettings.cmake
...

The second mention of atlas_export_sanitizer.cmake.in should've overwritten the file installed by the first line. As it was meant to install a different file. But it didn't for some reason that I don't understand just yet... 😦