Remove ContextManagerMock

[crazytonyli]

Changes

ContextManagerMock was created solely to workaround the deadlock issue mentioned in the saveContextAndWait override. Now that we've switched over to NSPersistentContainer(still needs to wait to see if it will introduce any issue in versions 21.1), we no longer need this "mock implementation". This PR deletes it and prepares for the follow up PR, which is deleting the legacy code and completing the NSPersistentContainer migration.

• This PR can be merged if NSPersistentContainer doesn't introduce major issue in v21.1.

Test instructions

All good if CI jobs pass.

Regression Notes

1. Potential unintended areas of impact
   None.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   None.

3. What automated tests I added (or what prevented me from doing so)
   None.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[wpmobilebot]

You can test the changes in Jetpack from this Pull Request by:

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• Then installing the build number pr19588-8136321 on your iPhone

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[wpmobilebot]

You can test the changes in WordPress from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr19588-8136321 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[mokagio]

Ready to merge once the preconditions have been validated 👍