Resolve "Uncheck `delete source branch` does nothing."

What does this MR do?

Fixes a simple logic error with displaying the delete status for Merge When Pipeline Succeeds.

See the before video below.

To test locally

Note: you will need a license that allows MWPS to test. I used an Ultimate license.

1. Create a project with a long-running pipeline or a pipeline that never succeeds.
2. Create an MR and select "Delete source branch when merge request is accepted." before saving.
3. Approve the MR, set MWPS and de-select the "Delete source branch"
4. The status should reflect that the source branch will not be deleted.

Screenshots (strongly suggested)

header	header
Screen_Recording_2021-04-09_at_8.26.18_PM	Screen_Recording_2021-04-09_at_8.29.37_PM

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
• Code review guidelines
• Merge request performance guidelines
• Style guides

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

Related to #215845 (closed)