Remove the index on Channels.DisplayName. (#8530)

As outlined in [this
discussion](https://pre-release.mattermost.com/core/pl/uw5bwmkb6irkbkn6pk9rkzpytr),
this index causes issues with MySQL's query planner, leading to full
table scans in a case where it would have made more sense to leverage a
filesort.

store/sqlstore/channel_store.go

@@ -90,7 +90,6 @@ func NewSqlChannelStore(sqlStore SqlStore, metrics einterfaces.MetricsInterface)
 func (s SqlChannelStore) CreateIndexesIfNotExists() {
 	s.CreateIndexIfNotExists("idx_channels_team_id", "Channels", "TeamId")
 	s.CreateIndexIfNotExists("idx_channels_name", "Channels", "Name")
-	s.CreateIndexIfNotExists("idx_channels_displayname", "Channels", "DisplayName")
 	s.CreateIndexIfNotExists("idx_channels_update_at", "Channels", "UpdateAt")
 	s.CreateIndexIfNotExists("idx_channels_create_at", "Channels", "CreateAt")
 	s.CreateIndexIfNotExists("idx_channels_delete_at", "Channels", "DeleteAt")