cleanup: general removal of TODO (and similar) comments

They tend to never get updated anyway so they're frequently inaccurate
and we never go back to revisit them anyway. We document issues to work
on properly in KNOWN_BUGS and TODO instead.

lib/smtp.c

@@ -1256,10 +1256,8 @@ static CURLcode smtp_done(struct connectdata *conn, CURLcode status,
 
     /* Run the state-machine
 
-       TODO: when the multi interface is used, this _really_ should be using
-       the smtp_multi_statemach function but we have no general support for
-       non-blocking DONE operations, not in the multi state machine and with
-       Curl_done() invokes on several places in the code!
+       this should be using the smtp_multi_statemach function but we have no
+       support for non-blocking DONE operations!
     */
     result = smtp_block_statemach(conn);
   }