Fix usages of the credential-reference schema in the Undertow subsystem

There are some issues with the current usages of the credential-reference schema in the Undertow subsystem:

• WFLY-12217 originally updated various subsystem schemas to reference the new XML schema for credential-reference (urn:wildfly:credential-reference:1.0). As part of this change, wildfly-undertow_9_0.xsd was updated to make use of this schema, as shown here (notice that the credential-reference schema was imported and the rest of the file was updated to make use of the credentialReferenceType provided by the schema):
  https://github.com/wildfly/wildfly/pull/12378/files#diff-c5cdb3e1bfa441b2673ee062fe51fbf73ceb7be8e88e5eed99a6fe9f17232c28

• WFLY-12173 then added wildfly-undertow_10_0.xsd but accidentally left out the credential-reference changes (it looks like the PR for this was merged very shortly after WFLY-12217 which explains why the new schema file might not have been copied from the right version of wildfly-undertow_9_0.xsd).

• WFLY-13369 then added wildfly-undertow_11_0.xsd by copying from the incorrect wildfly-undertow_10_0.xsd but corrected the latter file to use the urn:wildfly:credential-reference:1.0 schema again. However, the additional changes from wildfly-undertow_9_0.xsd that update the file to actually make use of the credentialReferenceType provided from the schema were accidentally left out.

As discussed with Flavia, the following two changes should be made to clean things up:

• Remove the credential-reference schema import from wildfly-undertow_10_0.xsd since it is unused.
• Make use of the imported credential-reference schema in wildfly-undertow_11_0.xsd and remove the redundant types