fix service category delete bug

[jehunte]

What does this PR (Pull Request) do?

• This PR fixes Cannot delete Service Categories on TP #4950

Which Traffic Control components are affected by this PR?

Traffic Portal

What is the best way to verify this PR?

Create a Service Category and verify that you can delete it

If this is a bug fix, what versions of Traffic Control are affected?

Next version to be released

The following criteria are ALL met by this PR

• This PR includes tests OR I have explained why tests are unnecessary
• This PR includes documentation OR I have explained why documentation is unnecessary
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
• This PR includes any and all required license headers
• This PR ensures that database migration sequence is correct OR this PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[mitchell852]

@jehunte - looks good but sorry to do this to you. can you add documentation for the DELETE method of api/3.0/service_categories. You can follow this example: https://traffic-control-cdn.readthedocs.io/en/latest/api/v3/topologies.html#delete

[jehunte]

@mitchell852 Added documentation!

[mitchell852]

lgtm