Remove CXXSTD for next-root6 build

[TimoWilken]

This seems to cause build failures for GEANT4 in the AliPhysics CI, like the following:

CMake Warning at cmake/Modules/G4BuildSettings.cmake:171 (message):
  The GEANT4_BUILD_CXXSTD argument is deprecated.

  Please use CMAKE_CXX_STANDARD to set the required C++ Standard.  The value
  supplied will be used to set CMAKE_CXX_STANDARD, but this behaviour will be
  removed in future releases.
Call Stack (most recent call first):
  cmake/Modules/G4CMakeMain.cmake:53 (include)
  CMakeLists.txt:50 (include)

CMake Error at cmake/Modules/G4CMakeUtilities.cmake:112 (message):
  Value '11' for variable CMAKE_CXX_STANDARD is not allowed

  It must be selected from the set: 17;20 (DEFAULT: 17)

Call Stack (most recent call first):
  cmake/Modules/G4BuildSettings.cmake:179 (enum_option)
  cmake/Modules/G4CMakeMain.cmake:53 (include)
  CMakeLists.txt:50 (include)

-- Configuring incomplete, errors occurred!