Revert "Merge branch 'prevent_diff_in_email' into 'master'"

What does this MR do and why?

Reverts Add setting to not display code diffs in MR rev... (!79229 - merged)

From !79229 (comment 836189318)

Directly related: #352543 (closed) Epic: #352541 (closed)

Why Revert?

The Code Review group is currently engaged on a very similar issue related to uploads in which we debated the use of a setting. Ultimately, we believe that if organizations are setting projects to private, they have that expectation for all behavior of items that might reach users via notifications. Forcing organizations to find and then utilize a setting for this doesn't make sense when they've already implied this behavior by setting a project to private.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.