Remove round tabs support from CTabRendering

Removes the CSS-based rounded tab rendering feature from CTabRendering.
This includes the swt-corner-radius CSS property, the CirclePart enum
and
circle drawing methods, the cornerSize field, and all round-tab
rendering
branches in drawTabBody, drawSelectedTab, and drawUnselectedTab.

Also removes the USE_ROUND_TABS preference, the corresponding UI
checkbox
in ViewsPreferencePage, and the CSSPropertyCornerRadiusSWTHandler class.

Fixes #3806

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: vogella

bundles/org.eclipse.e4.ui.css.swt/plugin.xml

@@ -87,14 +87,6 @@
                name="background-color">
          </property-name>
       </handler>
-      <handler
-            adapter="org.eclipse.e4.ui.css.swt.dom.CTabFolderElement"
-            composite="false"
-            handler="org.eclipse.e4.ui.css.swt.properties.custom.CSSPropertyCornerRadiusSWTHandler">
-         <property-name
-               name="swt-corner-radius">
-         </property-name>
-      </handler>
       <handler
             adapter="org.eclipse.e4.ui.css.swt.dom.CTabFolderElement"
             composite="false"

bundles/org.eclipse.e4.ui.css.swt/src/org/eclipse/e4/ui/css/swt/properties/custom/CSSPropertyCornerRadiusSWTHandler.java

@@ -38,8 +38,6 @@ public interface ICTabRendering {
 
 	void setOuterKeyline(Color color);
 
-	void setCornerRadius(int radius);
-
 	/**
 	 * Sets whether to use a custom tab background (reusing tab colors and
 	 * gradients), or default one from plain CTabFolder (using widget background