How to hide drupal error message - one or more problems were detected with your drupal installation
You could check the status report and try to resolve the error but I guess you are happy to leave it that way.

Or you could hide the message by selecting the class and choosing not to display it using CSS.
Create a file called style.css in your "Theme" folder (alongside the location of page.tpl.php) and place the class within it (you can check the class by using Firebug/Inspector "right click on it, then inspect element").

I think the class will be .warning or .alert, and an example CSS code would be:

div.warning, .alert, div.error, tr.error
{
display:none;
}


This would however hide all alerts and warnings for elements with the same class.  So be careful because you could miss other critical notifications.  You could have a style sheet for non-admin users that would turn this off and another for admin areas that would display these messages.  Its your choice.