message: Update eosio.system-unregprod-rc.md
  remove deprecated arbitration clause
diff: ### Description
  -The `{{ unregprod }}` action...
  +The `{{ unregprod }}` action unregisters a previously registered block producer candidate.
  ### Input and Input Type
  @@ -13,5 +13,3 @@ The `{{ unregprod }}` action requires the following `input` and `input type`:
  | `{{ unregprod }}` | `{{ producerVar }}` | `{{ account_name }}` |
  As an authorized party I {{ signer }} wish to unregister the block producer candidate {{ producerVar }}, rendering that candidate no longer able to receive votes.
  -
  -All disputes arising from this contract shall be resolved in the EOS Core Arbitration Forum.
  