Remove specific errors for very old obsolete syntax (#9627)

Mostly as per a short discussion on irc. (@cmr)

    08:46 < cmr> so I'm thinking
    Obsolete{Let,With,FieldTerminator,ClassTraits,ModeInFnType,MoveInit,BinaryMove,I
    mplSyntax,MutOwnedPointer,MutVector,RecordType,RecordPattern,PostFnTySigil,Newty
    pEnum,Mode,ImplicitSelf,LifetimeNotation,Purity,StaticMethod,ConstItem,FixedLeng
    thVectorType}
    08:46 < cmr> Those are the ones that are older than 0.6
    08:46 < cmr> (at least!)

This PR removes these specific "obsolete syntax"/"suggestion for change" errors and just lets the parser run into regular parser errors for long-invalid syntax. I also removed `ObsoletePrivSection` which apparently dates further back than cmr or I could recall and `ObsoleteUnenforcedBound` which seemed unused. Also I removed `ObsoleteNewtypeEnum`.