ChainSync client: remove redundant intersection check

[amesgen]

This step has caused confusions in the past.

See the commit description of the single commit.

[facundominguez]

Not sure if it is relevant, but in #1033 (3164620) I found a trace were the removed code sets the candidate fragment to G from G | 1-0. This this is the failure. And I'm attaching the trace where it can be seen that at tick 44, adversary 2 gets its candidate fragment set to G (line 2148). Commenting the transaction that sets the candidate fragment in nextStep' stops setting the candidate fragment to G (new trace)

cabal run ouroboros-consensus-diffusion:consensus-test -- -p '/stalling leashing attack/' --quickcheck-replay=967471

I'm not sure it is relevant because it could be a CSJ artifact.

[amesgen]

the removed code sets the candidate fragment to G from G | 1-0.

I don't think this can be the case; the code that is removed in this PR never changes the tip of the candidate; it only ever causes us to disconnect or to change the anchor of the candidate fragment.

[facundominguez]

I found a trace were the removed code sets the candidate fragment to G from G | 1-0

Today, with additional tracing, looks like this is not the case. In all likelihood, it is a false alarm.

[nfrisby]

Thanks for the simplification!