Remove System theme

The goal of the System theme was to fill the gap between default SWT
behavior and modern practices. But since such modern practices are
better to be built in SWT, it seems irrelevant and potentially harmful
to maintain the System theme as a workaround; efforts would be better
placed directly in SWT.

Author: mickaelistria

bundles/org.eclipse.ui.themes/css/e4_system.css

@@ -21,7 +21,6 @@ theme.mac = Light
 theme.win = Light
 theme.dark = Dark
 theme.high-contrast = High Contrast
-theme.system = System
 
 #New theme element definitions
 DARK_BACKGROUND=Dark Background Color

bundles/org.eclipse.ui.themes/css/system/e4-light-drag-styling.css

@@ -51,11 +51,6 @@
             label="%theme.win"
             os="win32">
       </theme>
-      <theme
-            basestylesheeturi="css/e4_system.css"
-            id="org.eclipse.e4.ui.css.theme.e4_system"
-            label="%theme.system">
-      </theme>
    <theme
 		basestylesheeturi="css/high-contrast.css"
         id="org.eclipse.e4.ui.css.theme.high-contrast"