Delete TestSRTPDrainLeak
Delete TestSRTPDrainLeak

This test was added to assert that rogue RTP/RTCP packets didn't cause
us to leak. The code that was actually causing the leak was removed,
so this isn't actually testing anything useful anymore

Resolves #823

pion/webrtc,pion/webrtc,pion/webrtc,pion/webrtc