Don't run the Title Generation Feature on attachments

[dkotter]

Description of the Change

Right now the Generate Titles button will show if you land on the attachment edit screen. At the moment, clicking the button won't work as expected so this PR removes that from showing if the post type is attachment. Also does some minor cleanup to the code to avoid nested conditionals.

We could look at adding a new setting to the Title Generation Feature to allow someone to choose which post types should work, similar to a few of our other Features but wanted a quicker fix for now.

Closes #995

How to test the Change

1. Enable the Title Generation Feature
2. Go to the edit screen for an attachment and ensure the Generate Titles button no longer shows
3. Enable the Classic Editor plugin
4. Edit a post and ensure the button shows and works

Changelog Entry

Fixed - Remove the Generate Titles button from attachments

Credits

Props @dkotter

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.

[iamdharmesh]

Looks good and tests well. Thanks for fixing this @dkotter