Remove `BuildsEmailService` records from `services` table

What does this MR do?

Records in the services table with type values of "BuildsEmailService" were migrated to be "PipelinesEmailService" in 12dd5ac2. The migration skipped migrating a "BuildsEmailService" record if its project already had a "PipelinesEmailService".

That commit also deleted the corresponding spec for the BuildsEmailService model and added a comment that the model could be removed in 9.1.

There are still 305 BuildsEmailService records in the GitLab.com database.

This MR:

1. Deletes records in the services table with type of "BuildsEmailService".
2. Deletes the model (which was recently renamed to Integrations::BuildsEmail in c1867b98)

• #331064 (closed)
• Fixes ~bug #332551 (closed)

Migration output

Up

This DELETE statement executes in 437.351 ms on #database-lab.

== 20210603222333 RemoveBuildsEmailServiceFromServices: migrating =============
-- execute("DELETE from services WHERE type = 'BuildsEmailService'")
   -> 0.0038s
== 20210603222333 RemoveBuildsEmailServiceFromServices: migrated (0.0039s) ====

Down

Note that this is a no-op:

== 20210603222333 RemoveBuildsEmailServiceFromServices: reverting =============
== 20210603222333 RemoveBuildsEmailServiceFromServices: reverted (0.0000s) ====

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• [-] 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.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #331064 (closed)