MuonMDT_CnvTools+MuonCnvToolInterfaces: const fixes.

Scott Snyderrequested to merge
ssnyder/athena:mdtconst2.MuonMDT_CnvTools-20210327 into master

Make interface of MsgCsmContByteStreamTool const. Remove duplicate declaration of IMDT_RDOtoByteStreamTool.

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