Remove obsolete build file

The build file  RecoHI/HiCentralityAlgos/test/BuildFile.xml was removed from 7_4_X a while back, and, for some unknown reason, this change did not propagate to 7_4_ROOT6_X.
This causes build warnings in this package, as the build file refers to files that have been removed.
Please merge this trivial request as soon as convenient.