Each vRA appliance builds out this file when you add a directory.
/usr/local/horizon/conf/domain_krb.properties
domain_krb.properties should list all domain controllers found by a DNS query. If you decommission domain controllers you will have to go into this file and remove the decommissioned domain controllers.
This is what the file looks like in my 7.5 lab
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#Wed Aug 22 21:27:11 UTC 2018
nanner.ninja=ad01.nanner.ninja\:389