[#384] eliminate vestigal `UNAUTHORIZED` paths in execute layer

• in MR !477 (merged) we introduced a centralized approach to handling and returning early from incoherent messages containing comamnds (including a sender issuing a command they are not authorized to issue). this is a better approach that is more cohesive and easy to reason about. yay!
• it also makes unnecessary the scattered authorization checks in every method in execute that might only be issuable by someone with a certain membership status. the previous MR removed the tests for those code paths but not the code paths.
• this MR removes the code paths (and cleans up execute a bit while we're at it. :))

Closes #384 (closed)