Quick flat state removal for non-tracked shard

[pugachAG]

The original idea was to use delete_range introduced in #8200 when cleaning flat state. That doesn't work due to sub-accounts for fixed shards.

With #8670 we can just remove all keys with a given shard_uid prefix.

[pugachAG]

@Longarithm let's consider the following layout with 3 shards:

ShardLayoutV1{
  fixed_shards: ["alice.near"],
  boundary_accounts: ["bob.near"],
  ..
}

What would the range we can use when cleaning shard with index 1 (with all keys before bob.near) without removing any subaccounts of alice.near (because those belong to shard with index 0)?

[Longarithm]

Hm I didn't realize that there is a plan to put subaccounts of a fixed account to the same shard. This is confusing because resharding of such shards will be computation-heavy. Asked in Zulip https://near.zulipchat.com/#narrow/stream/295558-pagoda.2Fcore/topic/fixed.20shards/near/320919706

[Longarithm]

Discussed offline: it is fine to ignore subaccounts feature because fixing single account is enough. Though we don't want to drop fixed accounts completely because in theory they could allow us to quickly move some large account to a separate shard.
We should mention this in comments to avoid future confusions.

[pugachAG]

Update: we've decided not to implement it for now.

This requires removing subaccounts support for accounts in fixed_shards which is a pretty significant change. Also this would be the first place in our codebase where we start relying on trie keys belonging to a single shard forming ranges, which introduces more assumptions to the systems and makes it prone to breaking in case of future changes. Considering that we don't find the underlying use case to be significant enough, so we are not proceeding with this for now.