ITL: Remove deprecated dns_expected_answer attribute

There's still support for the optional dns_expected_answers attribute
which supports an array as value.

fixes #4839

doc/10-icinga-template-library.md

@@ -321,7 +321,6 @@ Name                 | Description
 ---------------------|--------------
 dns_lookup           | **Optional.** The hostname or IP to query the DNS for. Defaults to "$host_name$".
 dns_server           | **Optional.** The DNS server to query. Defaults to the server configured in the OS.
-dns_expected_answer  | **Optional.** The answer to look for. A hostname must end with a dot. **Deprecated in 2.3.**
 dns_expected_answers | **Optional.** The answer(s) to look for. A hostname must end with a dot. Multiple answers must be defined as array.
 dns_authoritative    | **Optional.** Expect the server to send an authoritative answer.
 dns_wtime            | **Optional.** Return warning if elapsed time exceeds value.

itl/command-plugins.conf

@@ -1521,10 +1521,6 @@ object CheckCommand "dns" {
 			value = "$dns_expected_answers$"
 			description = "Optional ip address or host you expect the DNS server to return. Host must end with a dot (.). This option can be repeated multiple times (Returns OK if any value match). If multiple addresses are returned at once, you have to match the whole string of addresses separated with commas (sorted alphabetically)."
 		}
-		"-a_old" = {
-			key = "-a"
-			value ="$dns_expected_answer$"
-		}
 		"-A" = {
 			set_if = "$dns_authoritative$"
 		}
@@ -1543,7 +1539,6 @@ object CheckCommand "dns" {
 	}
 
 	vars.dns_lookup = "$host.name$"
-	vars.dns_expected_answer = "$check_address$"
 	vars.dns_timeout = 10
 }