MM-56878: Remove unused store method (#26230)

IsUserInChannelUseCache wasn't being used anywhere.

https://mattermost.atlassian.net/browse/MM-56878

```release-note
NONE
```

server/channels/store/timerlayer/timerlayer.go

@@ -1918,22 +1918,6 @@ func (s *TimerLayerChannelStore) InvalidatePinnedPostCount(channelID string) {
 	}
 }
 
-func (s *TimerLayerChannelStore) IsUserInChannelUseCache(userID string, channelID string) bool {
-	start := time.Now()
-
-	result := s.ChannelStore.IsUserInChannelUseCache(userID, channelID)
-
-	elapsed := float64(time.Since(start)) / float64(time.Second)
-	if s.Root.Metrics != nil {
-		success := "false"
-		if true {
-			success = "true"
-		}
-		s.Root.Metrics.ObserveStoreMethodDuration("ChannelStore.IsUserInChannelUseCache", success, elapsed)
-	}
-	return result
-}
-
 func (s *TimerLayerChannelStore) MigrateChannelMembers(fromChannelID string, fromUserID string) (map[string]string, error) {
 	start := time.Now()