DRY api/users_spec deactivation tests

What does this MR do?

Uses subject { post api("/users/#{user.id}/deactivate", api_user) } instead of use post multiple times.

Note: spec/requests/api/users_spec.rb:2452 context 'for an internal user' do is blocked by !43356 (merged), which changes the return code from 201 to 403. So currently it'll fail with expected the response to have status code :forbidden but it was 201. The response was: false but once !43356 (merged) lands I'll do a rebase and it should pass.

Similar to !44057 (merged)

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
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• 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
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