Remove specific errors for very old obsolete syntax

Replaces existing tests for removed obsolete-syntax errors with tests
for the resulting regular errors, adds a test for each of the removed
parser errors to make sure that obsolete forms don't start working
again, removes some obsolete/superfluous tests that were now failing.

Deletes some amount of dead code in the parser, also includes some small
changes to parser error messages to accomodate new tests.
auto merge of #9712 : ben0x539/rust/obsolete-syntax, r=pcwalton

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`.