Removed TODO for VDMs

From the USB PD spec, Revision 2.0, S. 6.4.4.1:

* A DFP or UFP which does not support Unstructured VDMs Shall Ignore
  any Unstructured VDMs received.

From S. 6.4.4.2:

* A DFP or UFP which does not support Structured VDMs Shall Ignore any
  Structured VDMs received.

Therefore, as long as we do not support any VDMs, we Shall Ignore them,
which is exactly what we're doing.  We're not doing anything wrong, so I
removed the TODO comment that implied we were doing something wrong.

src/policy_engine.c

@@ -216,8 +216,6 @@ static enum policy_engine_state pe_sink_ready(void)
     if (evt & PDB_EVT_PE_MSG_RX) {
         if (chMBFetch(&pdb_pe_mailbox, (msg_t *) &policy_engine_message, TIME_IMMEDIATE) == MSG_OK) {
             /* Ignore vendor-defined messages */
-            /* TODO: we're supposed to send NAKs in response to all
-             * unsupported structured VDMs except Attention. */
             if (PD_MSGTYPE_GET(policy_engine_message) == PD_MSGTYPE_VENDOR_DEFINED
                     && PD_NUMOBJ_GET(policy_engine_message) > 0) {
                 chPoolFree(&pdb_msg_pool, policy_engine_message);