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/telnet.c

@@ -1220,7 +1220,6 @@ CURLcode telrcv(struct connectdata *conn,
 }
 
 /* Escape and send a telnet data block */
-/* TODO: write large chunks of data instead of one byte at a time */
 static CURLcode send_telnet_data(struct connectdata *conn,
                                  char *buffer, ssize_t nread)
 {