Moved from local to LCG json library

Note that this depends on a fix in the LbDevTools toolchain (lhcb-core/LbDevTools!151 (merged)) for using properly INTERFACE libraries in cmake.

main/include/json.hpp was a copy of an external library. This commit drops it in favor of using the released version of it in LCG. It has the big advantage to be maintained and thus to be recent. The current copy was not and thus created conflicts with the usage of the same library inside Gaudi.