Removing/ Undoing Database addition to Alwayson Groups
Yes it is possible to do so. However, there are some possible issues you have to consider or expect. 


You will have at some down time in you application or web (whatever accessing the database). 
You need to change the connection string. AOA group connection string uses the listener or AoAgroup. When you switch back, you need to use regular connection string FQDN of your server or IP of your address. 
You might also need to remove the server from the cluster. 
When you failover databases under your secondary server remain under no recovery state. Databases in your primary server will be online but sometimes (especially if you have really big database) if there is some transaction log left and during the failover, your database will hang up under suspect mode.