lrwpan: Remove unnecessary Bcst filter from MAC

The present MR removes an unnecessary broadcast filter in the MAC PdDataIndication() function. The filter was initially there as a quick and dirty way to stop broadcasts from being received in the same device that issues them.

This, of course, should have never been filtered in the MAC, but at the time it was the only way to ensure this behavior. This problem was patched at some point in the previous release (Spectrum Changes?) and the filter is no longer necessary, in fact, it currently prevents the reception of some packets that should be otherwise received.

lr-wpan-mac-test.cc is also adjusted to enforce the checking of this correct behavior.

RELEASE_NOTES and CHANGES will be updated before the merge.