AthenaKernel: remove nlohmann dependency

Frank Winklmeierrequested to merge
fwinkl/athena:athkernel_nlohmann into master

The nlohmann dependency was introduced as a temporary work-around for Gaudi v34r1 in !37533 (merged). With the new cmake configuration of Gaudi this is no longer needed.

Also remove unused boost/program_options dependency.

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