Cleanup API GET service endpoint

What does this MR do?

TL;DR: It cleans up some leftovers of a feature that was removed.

The GET /projects/:id/services/:service_name endpoint used to return passwords for service integrations to admin users. This has been removed in a security fix. This MR removes some remaining code of that feature. We filter out passwords for all users now: https://gitlab.com/gitlab-org/gitlab/blob/master/app%2Fmodels%2Fservice.rb#L138-141

The changes should not affect the API behaviour. It is just code cleanup

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

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