fix random failures due to unnecessary headers on requests

was unintendedly replaying the save _H4 header on all requests, what was
causing random failures on responses from the API.

dnsapi/dns_linode_v4.sh

@@ -192,6 +192,10 @@ _rest() {
     response="$(_get "$LINODE_V4_API_URL$ep$data")"
   fi
 
+  # unset _H4, for it not to be used on every request unnecessarily, because it
+  # causes random failures inside Linode API when using unnecessary _H4 parameters (e.g. X-Filter)
+  unset _H4
+
   if [ "$?" != "0" ]; then
     _err "error $ep"
     return 1