Testing: Remove references to WordPressMocks repo

[rachelmcr]

We are no longer trying to keep the WordPressMocks repo updated with changes made on Android, since the API requests on the two platforms aren't identical and it's hard to keep the mocks in a state that works well on both platforms. This PR removes references to the WordPressMocks repo in the docs, to reflect that change.

Internal ref: paaHJt-PH-p2

PR submission checklist:

• I have considered adding unit tests where possible.
• 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.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

	Warnings
⚠️	PR is not assigned to a milestone.

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[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APK here.

[loremattei]

Many thanks for this @rachelmcr!