Disable Tests Fetcher

[DN6]

What does this PR do?

Test Fetcher appears to be working correctly, but I noticed that a bunch of unrelated tests are being fetched for PRs. There are a few imports causing this issue

1. The diffusers.pipelines.stable_diffusion.convert_from_ckpt is directly imported in the FromSingleFileMixin, triggering all tests for all pipelines using that Mixin whenever the StableDiffusionPipeline is changed.
2. StableDiffusionPipeline is directly imported into test_pipelines_common.py so all tests importing from this file are also triggered.
3. A change made to any of the pipelines imported in AutoPipeline also triggers tests for the other pipelines in auto pipeline.

Proposing to pause test fetcher for now until these import issues are resolved.

Fixes # (issue)

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