vi /etc/yum/pluginconf.d/subscription-manager.confコマンドにてenabled=1をenabled=0に変更します。 これによりsubscription-managerが無効化され、該当のメッセージが表示されなくなります。
# cat /etc/yum/pluginconf.d/subscription-manager.conf
[main]
enabled=1
# When following option is set to 1, then all repositories defined outside redhat.repo will be disabled
# every time subscription-manager plugin is triggered by dnf or yum
disable_system_repos=0
# vi /etc/yum/pluginconf.d/subscription-manager.conf
[main]
enabled=0
# When following option is set to 1, then all repositories defined outside redhat.repo will be disabled
# every time subscription-manager plugin is triggered by dnf or yum
disable_system_repos=0