Sharepoint - "The current page has been customized from its template. Revert to template."

The message means that you (or somebody else with permission) have made customizations on the page which deviates from the page template. This is a state of the page called un-ghosted.

If you don't want these customizations, simply click the link "Revert to template" and the page will go back to use its template (ghosted).

If you simply want to remove the message and keep customizations, you can hide it in css with the following

#pageStatusBar{
  height:0px; 
  font-size:0px; 
  padding:0px; 
  border-style:none;
}

or

#pageStatusBar{
    display:none!important;
}

all you need is...

#DeltaPageStatusBar {
  display: none;
}

in either your css or in style tags in your page.