Remove pipelineGraphLayersView feature flag [RUN AS-IF-FOSS] [RUN ALL RSPEC]

What does this MR do?

Removes the pipeline_graph_layers_view feature flag, which was defaulted on in %13.2. This is in preparation for removing the feature flag that gates the new graph.

Screenshots or Screencasts (strongly suggested)

No visual change.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)

Related to #328538 (closed)