amt: remove unnecessary locks

By the previous patch, amt gateway handlers are changed to worked by
a single thread.
So, most locks for gateway are not needed.
So, it removes.

Fixes: cbc21dc1 ("amt: add data plane of amt interface")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>