Simplify handling of PEP 517 metadata temp dir

[pradyunsg]

I think the diff is fairly self-explanatory?

• Not using temp_dir as a context manager here, so, the delete argument to __init__ has no effect and has been dropped.
• Use a better, more descriptive name for temporary directory
• Drop an unnecessary (and incorrect¹) InstallRequirement.cleanup

¹ self._temp_dir is not set anywhere except prepare_pep517_metadata, so it'll fail with an AttributeError for everything else.

[pradyunsg]

Thanks for the review @pfmoore!

By any chance, would you have time to take a look at #7087? (it's perfectly cool if you don't)