L1Decoder: Remove obsolete unit test

Rafal Bielskirequested to merge
rbielski/athena:l1decoder-rm-test into master

Removing a test which was running a full HLT menu athena job on 20 events, which is far from any definition of a "unit" test. The same is covered well by existing CI tests.

Also removing unused reference file.

cc @tbold

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