refactor: remove unused variable newChannelId2channel in InitChannelCache - #2385

Fixes #2321

Problem

In model/cache.go, the InitChannelCache function declares and populates a newChannelId2channel map but never reads from it afterward. This is dead code — the variable serves no purpose.

// Before: allocated and filled but never used
newChannelId2channel := make(map[int]*Channel)
for _, channel := range channels {
    newChannelId2channel[channel.Id] = channel
}

Solution

Remove the unused newChannelId2channel variable and its associated loop iteration. The remaining logic (building newGroup2model2channels and syncing it to group2model2channels) is unchanged.

Testing

No behaviour change — the removed code had no effect. Existing tests continue to pass.