Revert "handle empty objects in sync DELETEs"

[diracdeltas]

This reverts commit 91ef559 and pulls in fixes from the sync implementation in 0.17.x

fix #9724

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:
see #9724
bonus points: re-run test plan from brave/sync#111

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[diracdeltas]

syncing tests seem borked on this branch but they're also borked on master so probably not related to the changes

[ayumi]

#9724 success

brave/sync#111 success

🐓

[bsclifton]

@diracdeltas can you make sure this gets merged to 0.18.x also?

[diracdeltas]

yup