TransactionIDCache appears unused

[DMRobertson]

The only construction I can see is

sliding-sync/sync2/txnid.go

Lines 13 to 20 in 2920191

	func NewTransactionIDCache() *TransactionIDCache {
	c := ttlcache.NewCache()
	c.SetTTL(5 * time.Minute) // keep transaction IDs for 5 minutes before forgetting about them
	c.SkipTTLExtensionOnHit(true) // we don't care how many times they ask for the item, 5min is the limit.
	return &TransactionIDCache{
	cache: c,
	}
	}

which is only used by tests:

sliding-sync/sync2/txnid_test.go

Lines 5 to 13 in 2920191

	func TestTransactionIDCache(t *testing.T) {
	alice := "@alice:localhost"
	bob := "@bob:localhost"
	eventA := "$a:localhost"
	eventB := "$b:localhost"
	eventC := "$c:localhost"
	txn1 := "1"
	txn2 := "2"
	cache := NewTransactionIDCache()

Presumably it was supposed to be used to cache this query?

sliding-sync/sync3/handler/handler.go

Lines 533 to 539 in ca8a2d7

	func (h *SyncLiveHandler) TransactionIDForEvents(deviceID string, eventIDs []string) (eventIDToTxnID map[string]string) {
	eventIDToTxnID, err := h.Storage.TransactionsTable.Select(deviceID, eventIDs)
	if err != nil {
	logger.Warn().Str("err", err.Error()).Str("device", deviceID).Msg("failed to select txn IDs for events")
	}
	return
	}

We should determin if txn ID queries are heavy enough to need a cache, and then either use the cache or delete the dead code.