message: doc: remove obsolete `okSafeMode` RPC guideline from developer notes
  Since the flag has been removed from the RPC command table in commit
  (PR this guideline
  is not relevant anymore and can be removed.
diff: @@ -1159,13 +1159,6 @@ A few guidelines for introducing and reviewing new RPC interfaces:
  - *Rationale*: If not, the call can not be used with name-based arguments.
  -- Set okSafeMode in the RPC command table to a sensible value: safe mode is when the
  - blockchain is regarded to be in a confused state, and the client deems it unsafe to
  - do anything irreversible such as send. Anything that just queries should be permitted.
  -
  - - *Rationale*: Troubleshooting a node in safe mode is difficult if half the
  - RPCs don't work.
  -
  - Add every non-string RPC argument `(method, idx, name)` to the table `vRPCConvertParams` in `rpc/client.cpp`.
  - *Rationale*: `peercoin-cli` and the GUI debug console use this table to determine how to
  