MM-7188: Cleaning push notification on every read, not only on channel switch (#9348)

* MM-7188: Cleaning push notification on every read, not only on channel switch

* Removed unnecesary goroutine

* Fixing tests

* Applying suggestion from PR

app/channel_test.go

@@ -389,7 +389,7 @@ func TestAddChannelMemberNoUserRequestor(t *testing.T) {
 	channel := th.createChannel(th.BasicTeam, model.CHANNEL_OPEN)
 	userRequestorId := ""
 	postRootId := ""
-	if _, err := th.App.AddChannelMember(user.Id, channel, userRequestorId, postRootId); err != nil {
+	if _, err := th.App.AddChannelMember(user.Id, channel, userRequestorId, postRootId, false); err != nil {
 		t.Fatal("Failed to add user to channel. Error: " + err.Message)
 	}