CI: Clean up the [scheduled_extended_test] pipeline

Context

This pipeline has no need for the build jobs: they were used when we ran a subset of the python tests in the scheduled pipeline. The pytests have since been removed. Furthermore, they already run in the before_merging pipeline, ensuring that no code that does not compile lands on master. However, I've kept the build_arm64 job since it is manual in both MR and master pipeline. I recently broke that job (!9178 (merged)), so I think it makes sense to run it once in a while.

The ometric_default and the coverage_default jobs only need to run in master_branch pipelines (which they already do), to provide a base line for before_merging pipelines.

Consequently this MR removes these jobs from the scheduled pipeline.

Manually testing the MR

As described here, I've created an "artificial" scheduled pipeline here. You can do the same by following the instructions in !8942 (merged), replacing the pipeline type master_branch by schedule_extended_test in the call to run_pipeline_type.sh.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR