Retire high contrast theme

The High Contrast theme does look very bad, it is better to retire it as
this is clearly not maintained.

Fixes #3095

Author: jnerlich

bundles/org.eclipse.ui.themes/css/high-contrast.css

@@ -51,11 +51,6 @@
             label="%theme.win"
             os="win32">
       </theme>
-   <theme
-		basestylesheeturi="css/high-contrast.css"
-        id="org.eclipse.e4.ui.css.theme.high-contrast"
-        label="%theme.high-contrast">
-   </theme>
 
    <themeAssociation
 		themeId="org.eclipse.e4.ui.css.theme.e4_classic"