Drop show_diff_preview_in_email column

What does this MR do and why?

As outlined in #352541 (closed), this is step 2 of reverting !79229 (merged)

@kerrizor: Note that this is a copy/recreation of !80445 (closed), which encountered a strange error in the DB pipelines that we couldn't resolve, except to recreate a new migration.

Database

Up

== 20220208184958 RemoveShowDiffPreviewInEmailColumn: migrating ===============
-- remove_column(:project_settings, :show_diff_preview_in_email, :boolean)
   -> 0.0020s
== 20220208184958 RemoveShowDiffPreviewInEmailColumn: migrated (0.0021s) ======

Down

== 20220208184958 RemoveShowDiffPreviewInEmailColumn: reverting ===============
-- add_column(:project_settings, :show_diff_preview_in_email, :boolean)
   -> 0.0024s
== 20220208184958 RemoveShowDiffPreviewInEmailColumn: reverted (0.0055s) ======

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.

Related to #352544 (closed)