[Gutenberg] Remove image count from gallery processor

[mkevins]

Description

This PR removes the imageCount attribute from the gallery block upload completion processor (and the test content). Originally, this attribute was used to return early from the regular processor, and signal a subsequent recursion to handle inner blocks. It turns out that the absense of the ids attribute is sufficient to accomplish the same. This PR removes the cruft (since this attribute has now been removed from the block schema), and updates the test content to reflect the latest format.

To test:
Run the unit tests for the upload completion processors.

Manual test:
Test the upload flow in this test with the refactored version of the gallery.

Regression Notes

1. Potential unintended areas of impact
   Uplodate completion processors

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Updated unit tests for the change in content

3. What automated tests I added (or what prevented me from doing so)
   N/A

PR submission checklist:

• I have completed the Regression Notes.
• 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]

You can test the changes on this Pull Request by downloading the APKs:

• Jetpack-pr-15196-build-114189.apk
• WordPress-pr-15196-build-114189.apk

[antonis]

Great work @mkevins 👍
The code changes LGTM and the app works as expected 🎉

[mkevins]

Thanks Antonis for testing and reviewing. 😃