[domain@localhost:9999 /] /server-group=main-server-group/deployment=postgresql-9.0-801.jdbc4.jar:remove
{
"outcome" => "success",
"result" => {"server-groups" => [("main-server-group" => {"master" => {
"host" => "master",
"response" =>

}})]}
}
[domain@localhost:9999 /] /profile=default/subsystem=datasources/data-source=postgres:read-resource
{
"outcome" => "success",
"result" => {
"background-validation" => undefined,
"background-validation-minutes" => undefined,
"blocking-timeout-wait-millis" => undefined,
"connection-url" => "jdbc:postgresql://127.0.0.1:5432/rhqdev",
"driver-name" => "postgresql-9.0-801.jdbc4.jar",

Console says
[Server:server-two] 13:55:09,768 INFO [org.jboss.as.server.controller] (pool-1-thread-4) Undeployed "postgresql-9.0-801.jdbc4.jar"
[Server:server-two] 13:55:09,769 INFO [org.jboss.as.controller] (pool-1-thread-4) Service status report
[Server:server-two] New missing/unsatisfied dependencies:
[Server:server-two] service jboss.jdbc-driver.postgresql-9_0-801_jdbc4_jar (missing)

and after reboot:

[Server:server-one] 13:58:20,938 INFO [org.jboss.as.controller] (Controller Boot Thread) Service status report
[Server:server-one] New missing/unsatisfied dependencies:
[Server:server-one] service jboss.jdbc-driver.postgresql-9_0-801_jdbc4_jar (missing)

I think removing the driver from a server group should not be possible if a datasource still refers to it.
Basically the same way I can't remove a deployment from /deployment, while still in use in /server-group=x/deployment.