Remove note about Stream/Serial deprecation

Author: Jenny Plunkett

drivers/Serial.h

@@ -82,10 +82,6 @@ class Serial : public SerialBase, public Stream, private NonCopyable<Serial> {
      */
     Serial(PinName tx, PinName rx, int baud);
 
-    /* Stream gives us a FileHandle with non-functional poll()/readable()/writable. Pass through
-     * the calls from the SerialBase instead for backwards compatibility. This problem is
-     * part of why Stream and Serial should be deprecated.
-     */
     bool readable()
     {
         return SerialBase::readable();