fix: Compilation error on macOS `Undefined symbol: OBJC_CLASS$_PFProductsRequestHandler`

[dplewis]

New Pull Request Checklist

• [ ] I am not disclosing a vulnerability.
• [ ] I am creating this PR in reference to an issue.

Issue Description

PFProduct and PFPurchase aren't supported on MacOS

Closes: https://github.com/parse-community/Parse-SDK-iOS-OSX/issues/1699

Approach

• Remove PFPurchaseController from Mac build

TODOs before merging

• [x] Add tests
• [x] Add changes to documentation (guides, repository pages, in-code descriptions)

[parse-github-assistant[bot]]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (2316a3f) 77.86% compared to head (1975c55) 78.22%. Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1739 +/- ## ========================================== + Coverage 77.86% 78.22% +0.35% ========================================== Files 299 307 +8 Lines 36328 36981 +653 ========================================== + Hits 28288 28927 +639 - Misses 8040 8054 +14 ``` | [Files](https://app.codecov.io/gh/parse-community/Parse-SDK-iOS-OSX/pull/1739?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=parse-community) | Coverage Δ | | |---|---|---| | [...nternal/Purchase/Controller/PFPurchaseController.m](https://app.codecov.io/gh/parse-community/Parse-SDK-iOS-OSX/pull/1739?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=parse-community#diff-UGFyc2UvUGFyc2UvSW50ZXJuYWwvUHVyY2hhc2UvQ29udHJvbGxlci9QRlB1cmNoYXNlQ29udHJvbGxlci5t) | `88.81% <ø> (ø)` | | ... and [9 files with indirect coverage changes](https://app.codecov.io/gh/parse-community/Parse-SDK-iOS-OSX/pull/1739/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=parse-community)

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[mtrezza]

I've changed the PR title, could you check whether it's correct? As a reminder, the PR title will be part of the changelog and should describe what the bug is, rather than what the solution or fixed state is.

[dplewis]

That is correct

[parseplatformorg]

🎉 This change has been released in version 2.7.1