Remove all safetensors import mocking from tests

[patrickvonplaten]

We have a use_safetensors argument in from_pretrained which can fully control whether we load in safetensors or not. We can clean up this dummy import mess a bit here:

diffusers/tests/pipelines/test_pipelines.py

Line 399 in 95b7de8

diffusers.utils.import_utils._safetensors_available = safe_avail

and just work with use_safetensors instead which was added a while back: https://huggingface.co/docs/diffusers/main/en/api/diffusion_pipeline#diffusers.DiffusionPipeline.from_pretrained.use_safetensors

[patrickvonplaten]

@DN6 this could be interesting for you

[DN6]

Handled in PRs:
#4243
#4235