res/res_rtp_asterisk: clear smoother when local bridging

p2p_write updates txformat but doesn't require a smoother.  If a smoother
was created by another bridge type the smoother could fall out of date causing
one way audio issues.  To prevent this the smoother is now destroyed on the
start of native bridge.

ASTERISK-28284 #close

Change-Id: I84e67f144963787fff9b4d79ac500514fb40cdc6