refactor: remove dead code in cacheMergeIterator Domain()

[facundomedica]

Description

TL;DR: because the parent and the cache's domain is always the same, there's no point in doing these comparisons.

Here both parent and cache are created using the same input start and end:

cosmos-sdk/store/cachekv/store.go

Lines 179 to 196 in 252c673

	func (store *Store) iterator(start, end []byte, ascending bool) types.Iterator {
	store.mtx.Lock()
	defer store.mtx.Unlock()
	var parent, cache types.Iterator
	if ascending {
	parent = store.parent.Iterator(start, end)
	} else {
	parent = store.parent.ReverseIterator(start, end)
	}
	store.dirtyItems(start, end)
	cache = newMemIterator(start, end, store.sortedCache, store.deleted, ascending)
	return newCacheMergeIterator(parent, cache, ascending)
	}

The parent seems to always just take the start and end, store it in the iterator struct and then return it in its own Domain():

https://github.com/tendermint/tm-db/blob/b599fa5d668d5a68812ec5ebeccb9eb05235bb3a/goleveldb_iterator.go#L41-L53

https://github.com/tendermint/tm-db/blob/b599fa5d668d5a68812ec5ebeccb9eb05235bb3a/prefixdb_iterator.go#L57-L69

https://github.com/tendermint/tm-db/blob/b599fa5d668d5a68812ec5ebeccb9eb05235bb3a/rocksdb_iterator.go#L43-L55

And the cache (cache = newMemIterator(start, end, store.sortedCache, store.deleted, ascending)) does exactly the same (uses tm-db.MemDB):

https://github.com/tendermint/tm-db/blob/b599fa5d668d5a68812ec5ebeccb9eb05235bb3a/memdb_iterator.go#L34-L43

https://github.com/tendermint/tm-db/blob/b599fa5d668d5a68812ec5ebeccb9eb05235bb3a/memdb_iterator.go#L114-L117

If I'm not missing anything we can just use the parent or cache's domain indistinctly.

---

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[codecov]

Codecov Report

Merging #13433 (0f056b0) into main (facac1c) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13433      +/-   ##
==========================================
- Coverage   53.98%   53.98%   -0.01%     
==========================================
  Files         641      641              
  Lines       55032    55020      -12     
==========================================
- Hits        29711    29704       -7     
+ Misses      22936    22934       -2     
+ Partials     2385     2382       -3     

Impacted Files	Coverage Δ
store/cachekv/memiterator.go	93.54% <100.00%> (+0.21%)	⬆️
store/cachekv/mergeiterator.go	88.97% <100.00%> (+2.39%)	⬆️
crypto/keys/internal/ecdsa/privkey.go	81.13% <0.00%> (-1.89%)	⬇️
x/group/keeper/keeper.go	56.64% <0.00%> (+0.39%)	⬆️

[alexanderbez]

Nice! nit: changelog