Drop tsk-42: assigned property already implemented

The cross-namespace assignment command in question is `tsk export`,
which (since the export/accept feature commits) sets
`assigned=[[<target-ns>/tsk-N]]` on the source after a successful
write. With tsk-41's namespaced-link parser landed, the value also
renders with a numbered superscript in `tsk show` and is followable
via `tsk follow -l N`. The accept side strips `assigned` from the
copied attrs so the new local task isn't itself flagged as assigned.

No code change needed; this commit is just the bookkeeping drop.