You can remove it from the templates or override the cAcpNotificationBanner class from your stylesheet.
Removing it by overriding the cAcpNotificationBanner class:
Open your custom.css and paste this in it:
.cAcpNotificationBanner_information {
display: none;
}
Removing it from the templates (I don't recommend doing so):
Open the globalTemplate of your theme and search for {template="updateWarning" app="core" group="global" params=""}
You can remove it but this template holds another important template which is also shown to normal members, so instead of just removing it, we are going to replace it with: {template="announcementTop" app="core" group="global" params=""} and it's done.