remove dead deepfloyd if shared test mixin

[akshan-main]

What does this PR do?

Removes the IFPipelineTesterMixin from the deepfloyd_if test package __init__.py. All six IF test files migrated to the new mixin structure with their components inlined (#14580, #14596, #14602, #14603, #14604, #14606), so nothing imports it anymore. Follows #14606.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue or the forum? Discussed on Slack with @sayakpaul
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@sayakpaul

[sayakpaul]

Superb!