Remove unused type temp_working_dirt

[hannes-steffenhagen-diffblue]

This type has been added in 2012 and is presently unused. In its present state
it is difficult to use as it doesn't permit any way to recover from potential
errors (it is instead asserted that no errors can happen, which is not true).

Should functionally like this be needed in the future, I'd recommend a wrapper
function like

template<typename Closure>
void run_with_different_working_dir(
  const std::string& working_dir, Closure closure);

Which would change the working directory, execute the closure and change the
working directory back afterwards, throwing a system_exceptiont on failure.

• Each commit message has a non-empty body, explaining why the change was made.
• My contribution is formatted in line with CODING_STANDARD.md.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[tautschnig]

Nit picks: 1) in the commit message: s/functionally/functionality/ and 2) remove the blank line at the end of tempdir.cpp

[allredj]

This PR failed Diffblue compatibility checks (cbmc commit: 8264abf).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/86163537
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

[allredj]

Passed Diffblue compatibility checks (cbmc commit: 77afd48).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/86164156

[hannes-steffenhagen-diffblue]

@tautschnig Fixed the type and removed the extra line

[allredj]

Passed Diffblue compatibility checks (cbmc commit: b48381d).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/86170412

[hannes-steffenhagen-diffblue]

AWS fails due to seemingly unrelated timeout error. I've restarted the build.

[hannes-steffenhagen-diffblue]

I did retry the CodeBuild thing and it passed, but it didn't update the status in here. Not sure what's going on with that.

[tautschnig]

Maybe we can merge some other PR so that you can rebase this one ;-)

[tautschnig]

@hannes-steffenhagen-diffblue You can now do a rebase.

[hannes-steffenhagen-diffblue]

@tautschnig done

[allredj]

Passed Diffblue compatibility checks (cbmc commit: e2fc031).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/86268564