Remove any vestigial mentions of calling convention

[mpeck]

The database still contains a field for 'execution' which I believe refers to the old 'calling_convention' bits. We need to verify that all vestigial mentions of calling convention have been removed from cog.

parent #399

[vanstee]

Items to cleanup:

• Remove execution field from command table and model
• Remove once case form Pipeline.Planner
• Remove once/multi docs from commands that mention them
• Fixup tests that callout once/multi
• Remove from documentation