inspect program: command to remove fields from sourcedicts

[olorin]

This is currently difficult to do without writing a custom application for it; all existing utilities treat sourcedicts as append-only (semantically, not just under the hood). I propose adding a command to the 'inspect' program which takes an origin plus one or more field names and iterates over the contents list for that origin, sending deletion updates for that field for each sourcedict which has it set.