Drop ci_build_runtime_environments table

What does this MR do and why?

Drop ci_build_runtime_environments table

The table was replaced by p_ci_job_runtime_environments and is no longer referenced by any application code. Remove the loose foreign key watchers and move the table doc to deleted_tables.

References

MR that replaces ci_build_runtime_environments with p_ci_job_runtime_environments - !244035 (comment 3637216686)

Epic- Resumable Jobs for CI and Agent Sessions (gitlab-org#21159) • Vishal Tak, Ashvin Sharma • 19.4

MR acceptance checklist

1. How the data could be recovered in the event of an incident.
   1. There is no data because this table was never used in production.
2. The approximate number of records being affected.
   1. No data in the table
3. A description of the user experience impact.
   1. No data in the table, zero user experience impact.

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