Cleanup of ci_write_pipeline_variables_artifact feature flag

What does this MR do and why?

Background

Currently pipeline variables take up a lot of space in the database table p_ci_pipeline_variables. Our goal in the epic Implementation of offloading pipeline_variables... (gitlab-org#19989) is to offload this data to object storage by leveraging our existing pipeline artifacts framework to store pipeline variables as encrypted JSON artifacts.

As par of #580107 (closed):

In !221972 (merged), we introduced the ci_write_pipeline_variables_artifact feature flag to dual-write pipeline variables as encrypted artifacts via Ci::PipelineVariablesArtifactBuilder hooked into Chain::Build::Associations. We have rolled it out gradually and reached 100% of production.

This MR

This MR removes the ci_write_pipeline_variables_artifact feature flag now that the dual-write behaviour has been validated in production and deemed stable.

The following changes are included:

• PipelineVariablesArtifactBuilder is called unconditionally in Chain::Build::Associations#assign_pipeline_variables
• variables_attributes is always passed to CreatePipelineService in Gitlab::Chat::Command#create_pipeline
• The now not in use build_environment_variables method is removed from Gitlab::Chat::Command as it was only ever called behind the Feature.disabled? guard
• Updated specs

References

• FF rollout issue: [FF] : enable `ci_write_pipeline_variables_arti... (#580472 - closed)
• Part of epic: gitlab-org#19989
• FF introduced in: !221972 (merged)

Related to #580472 (closed)