ITS: remove shipped nlohmann_json

[f3sch]

Needs alisw/alidist#5885

ITS included a header only of the library but essentially did not depend on it for a while now.
However, some TRD code did explicitly. This adds the library as a dependency to O2 (which it anyways was through root and onnx), removes the header file and rectifies the include in trd.

[github-actions]

REQUEST FOR PRODUCTION RELEASES:
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This will add <label1> and <label2> and removes <label3>.

The following labels are available
async-2023-pbpb-apass4
async-2023-pp-apass4
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async-2024-PbPb-apass1
async-2024-ppRef-apass1
async-2024-PbPb-apass2
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[alibuild]

Error while checking build/O2/fullCI_slc9 for f6e5460 at 2025-05-21 01:51:

## sw/BUILD/O2-latest/log
/sw/SOURCES/O2/14295-slc9_x86-64/0/Detectors/TRD/workflow/include/TRDWorkflow/TRDEventDisplayFeedSpec.h:13:10: fatal error: nlohmann/json.hpp: No such file or directory
/sw/SOURCES/O2/14295-slc9_x86-64/0/Detectors/TRD/workflow/include/TRDWorkflow/TRDEventDisplayFeedSpec.h:13:10: fatal error: nlohmann/json.hpp: No such file or directory
ninja: build stopped: subcommand failed.

Full log here.

[alibuild]

Error while checking build/O2/fullCI_slc9 for 667a4cd at 2025-05-21 16:41:

## sw/BUILD/O2-latest/log
/sw/SOURCES/O2/14295-slc9_x86-64/0/Detectors/TRD/workflow/include/TRDWorkflow/TRDEventDisplayFeedSpec.h:13:10: fatal error: nlohmann/json.hpp: No such file or directory
ninja: build stopped: subcommand failed.

Full log here.