migrate ideogram4 lora tests to the pipeline-level mixins

[akshan-main]

What does this PR do?

Migrates the Ideogram4 pipeline LoRA tests from tests/lora/test_lora_layers_ideogram4.py to the pipeline-level mixins from #14268, same shape as #14613/#14615: TestIdeogram4PipelineLoRA(Ideogram4PipelineTesterConfig, LoraTesterMixin) and TestIdeogram4PipelineLoRAMemory(Ideogram4PipelineTesterConfig, LoraMemoryTesterMixin) in tests/pipelines/ideogram4/test_pipeline_ideogram4.py, and the old file removed.

The old file carried no Ideogram4-specific tests, so nothing needed porting: text-encoder tests skip through _lora_loadable_modules (transformer only) and the two block-scale skips disappear with UNetLoraTesterMixin. Follows #14615.

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