Remove bridges zombienet tests from CI (#3071)

closes https://github.com/paritytech/parity-bridges-common/issues/2796

This partially reverts the #2439 - there are some changes (unrelated to
CI) that we still want to keep. The reason of that removal is that with
async backing enabled for Rococo AH (and for other chains in the near
future), we see a lot of issues there (because we run `14` nodes +
additional standalone process within a same container and it causes a
lot of timeouts). There's no way known to me to fix it right now, so
we're removing those tests hopefully temporarily to keep CI green