Changes

mgw: Drop own MGCP extension 'noanswer'

This was added in e3d16bb77530f2b3f08fdaf9640dd98bd9e730fa 14 years ago:
"""
[mgcp] Protocol extension to not generate answers.

    For the NAT we want to send requests in a send and forget
    way and we are not interested in seeing the answers, so tell
    the gateway to not answer them.
"""

So this code is most probably related to osmo-bsc_mgcp time, and we
don't really need this feature in osmo-mgw, it really makes no sense.
Drop it.

Change-Id: Ica2d3d54cda41e0c6416913ab056843a0cd80c17