[MM-24065] Guest users should be able to search by channel (#14302)

* Remove unnecessary check for PERMISSION_LIST_TEAM_CHANNELS

In the autocompleteChannelsForTeamForSearch method we're checking for
the PERMISSION_LIST_TEAM_CHANNELS permission in order to avoid filtering
channels in the autocomplete search but this check is not necessary.

Now we're going directly to the database to search for those channels
in this specific method and we're filtering by channel membership and
team so there is no chance that we are going to filter undesired
channels to the user.

[Here](https://github.com/mattermost/mattermost-server/blob/v5.22.0/store/sqlstore/channel_store.go#L2014)
is the query where you can see the filtering we're making

api4/channel.go

@@ -865,11 +865,6 @@ func autocompleteChannelsForTeamForSearch(c *Context, w http.ResponseWriter, r *
 		return
 	}
 
-	if !c.App.SessionHasPermissionToTeam(*c.App.Session(), c.Params.TeamId, model.PERMISSION_LIST_TEAM_CHANNELS) {
-		c.SetPermissionError(model.PERMISSION_LIST_TEAM_CHANNELS)
-		return
-	}
-
 	name := r.URL.Query().Get("name")
 
 	channels, err := c.App.AutocompleteChannelsForSearch(c.Params.TeamId, c.App.Session().UserId, name)
@@ -878,8 +873,6 @@ func autocompleteChannelsForTeamForSearch(c *Context, w http.ResponseWriter, r *
 		return
 	}
 
-	// Don't fill in channels props, since unused by client and potentially expensive.
-
 	w.Write([]byte(channels.ToJson()))
 }