DEFAULT_TMPDIR is documented, but doesn't exist

The user's guide claims

    Even better idea: Set the :envvar:`DEFAULT_TMPDIR` :command:`make` variable when
    building GHC, and never worry about :envvar:`TMPDIR` again. (see the build
    documentation).

But only vestiges of this functionality remain in mk/config.mk.in and in nofib, and it doesn't appear to actually do anything. I suggest removing it from the documentation.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Documentation
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