Delete unclear paragraph from STP_OnBpduReceived.html

[adigostin]

Delete this:
"It is recommended that the application, before passing a received BPDU frame to the STP library, performs a check for malformed frames by comparing the value in the EtherType/Size field of the frame to the length of the received frame. Such malformed frames should be discarded, not passed to the STP library."

The library checks the BPDU anyway, and it is also unclear in this paragraph what the app is supposed to do with regard to short frames padded to 64 bytes.