Revert "Dgit: Introduce in_workarea and fresh_workarea"

This reverts commit aa073c5bdde68a1ab5026ba4daaf29c8bf361532.

These functions are not used on this branch.  I originally put them
here because I thought dgit would want to use them, but actually it
doesn't really.

There are no tests for them here and we are about to engage in
refactoring which will probably accidentally break them.  Instead,
delete them.

This revert should itself be re-reverted when these functionsn are
going to be used.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>