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/store.go

@@ -232,7 +232,6 @@ type ChannelStore interface {
 	GetAllChannelMembersForUser(userID string, allowFromCache bool, includeDeleted bool) (map[string]string, error)
 	GetChannelsMemberCount(channelIDs []string) (map[string]int64, error)
 	InvalidateAllChannelMembersForUser(userID string)
-	IsUserInChannelUseCache(userID string, channelID string) bool
 	GetAllChannelMembersNotifyPropsForChannel(channelID string, allowFromCache bool) (map[string]model.StringMap, error)
 	InvalidateCacheForChannelMembersNotifyProps(channelID string)
 	GetMemberForPost(postID string, userID string, includeArchivedChannels bool) (*model.ChannelMember, error)