Remove the databases from the server2 ( you can do this from Server1)
Then remove Server 2 from the DAG:
If it wont let you, remove via the configuration switch:
Example:
Remove-DatabaseAvailabilityGroupServer -Identity DAG2 -MailboxServer Server2 -ConfigurationOnly
Now remove Server1 from the DAG as well using the steps above if you want.
The DAG can remain as is with no members