ByteStreamStoragePlugins: Remove dcap protocol support

Remove the dcap I/O protocol support for dCache storage in ByteStreamStoragePlugins. The dcap library is currently not in dev4/LCG CentOS9 nightlies anymore and dcap is in principle deprecated over the root (xrootd) protocol since a long time. Relates to the discussion about setting up a CentOS9 build machine in ATLINFR-4538

Just to make sure that nothing unexpected breaks, using full-build/full-unit-tests.