Tidup WebHook specs

What does this MR do?

This MR makes two tweaks to the WebHook specs that were added in !66876 (merged).

We intended to assert records weren't updated using QueryRecorder rather than be_persisted: !66876 (diffs, comment 636790953)

It also removes an unneeded aggregate_failures.

Does this MR meet the acceptance criteria?

Conformity

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• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.