Remove usage of "extern" ByteStream_StorageType

Frank Winklmeierrequested to merge
fwinkl/athena:bytestream_storagetype_misc into master

This is a follow-up MR to !21339 (merged) in order to remove the usage of the extern symbol ByteStream_StorageType from FWD, ZDC and LUCID bytestream converters.

Also add override and remove one unnecessary virtual.

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