Drop tmp index on emails table again

What does this MR do?

This MR drops the temporary index in the emails table again. The index was dropped in !50981 (merged) however for some reason (probably a security MR merge) the index definition was re-introduced in the structure.sql file. (!50981 (comment 482845302))

Fixing the structure.sql file is not enough: developers who recreated the schema recently will have the index so we make to ensure it's gone.

This MR will not alter the state of the DB (except for devs) so I think we can omit the changelog.

Migration

Up (index is not there):

== 20210112084512 DropTmpIndexOnEmailsAgain: migrating ========================
-- transaction_open?()
   -> 0.0000s
-- indexes(:emails)
   -> 0.0026s
-- current_schema()
   -> 0.0002s
== 20210112084512 DropTmpIndexOnEmailsAgain: migrated (0.0057s) ===============

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team