Remove unneeded check for merge_request.project

What does this MR do?

This removes an unneeded check discovered while working on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14886#note_204611783.

I verified that we call this when an MR is successfully created / updated so this check isn't needed.

This line can be traced back to 3c867dfa which I think was the first ever iteration for MR hooks.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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

Closes #66368 (closed)