Remove unused wheel_cache argument in freeze operation

[sbidoul]

There is no reason why the freeze operation would use a WheelCache, and indeed this argument is unused. So I remove it.

[pfmoore]

A quick note. Pip 21.0 is due for release in about 2 weeks (see #9282). This item is on the 21.0 milestone, so it either needs to be implemented and merged to master before then, or it will miss the release (and I'll move it to the 21.1 milestone).

[sbidoul]

@pfmoore this is ready to merge. BTW, I generally refrain to merge my own PRs. Do we have a policy about that (number of approvals, etc), or is it left to the appreciation and common sense of pip committers (which is fine with me too) ?

[sbidoul]

That said, I'm going ahead and merging this as it is quite trivial.

[pfmoore]

@sbidoul I tend to merge my own PRs once they have approvals from other core devs (or if they are sufficiently simple and I'm confident in my own work - not true that often!), and I tend not to merge other core devs PRs in case they want to do so themselves. It's not a policy, though, just how I prefer to work.

If you ever want someone to do a merge for you, even if just to cast a last extra pair of eyes over a PR, then feel free to ping me. When merging another core dev's code, I tend to just do "did it pass CI, are there any outstanding review comments" so it's not a big problem for me to push the button.