eth/downloader: fix downloader cancel

[rjl493456442]

This PR reverts the changes brought by #32067

Previously, the chain insertion will always be interrupted once the downloader.Cancel is called.
Unfortunately, the aborted error from Blockchain will be propagated and ultimately leading to
the corresponding peer serving this chain segment being dropped.

Therefore, this PR reverts the behavioral changes and only interrupt the chain insertion when
tearing down Geth. The debug_setHead needs to be improved with better solution.

[rjl493456442]

The change is probably not correct. It's true that the aborted error will be bubbled up, but it's discarded by downloading without propagating to the handler.

The disconnection should be relevant with some other reasons.

[rjl493456442]

After some investigation, the peer dropping is caused by the skeleton header fetching.