Deprecate run_instance and remove unnecessary code

This begins a deprecation path for the compute manager run_instance
code.

For now the scheduler run_instance rpc method is being removed as it is
unused.

In the K cycle the scheduler schedule_run_instance methods can be
removed which means that the run_instance compute rpc method can be
removed.

In the L cycle the run_instance compute method can be removed.

This also removes cells schedule_run_instance methods which have been
unused since Havana.

bp remove-cast-to-schedule-run-instance

Change-Id: I284ac124fe2ec952db0feab61206eda2a20e40c8
Fix docstring typo in need_legacy_block_device_info

Change-Id: I9aaec5f73f3001c0926aba2fba4165f3a66319f1
Merge "Fix docstring typo in need_legacy_block_device_info"
Merge "Deprecate run_instance and remove unnecessary code"