Remove deprecated show-close CSS support

[vogella]

Since 2012 we are issuing a warning if the deprecated swt-close CSS property is used. We should remove this property.

[elsazac]

@vogella - could you pls elaborate a little on this? where do I see the warning?

[vogella]

That @elsazac for showing interest in this issue.

In org.eclipse.e4.ui.css.swt we have

      <handler
            adapter="org.eclipse.e4.ui.css.swt.dom.CTabItemElement"
            composite="false"
            handler="org.eclipse.e4.ui.css.swt.properties.custom.CSSPropertyShowCloseHandler">
         <property-name
               name="swt-show-close">
         </property-name>
             <property-name
               name="show-close" deprecated="renamed as swt-show-close">
         </property-name>
      </handler>

And the code in CSSPropertyShowCloseHandler, CTabFolderTest, CTabItemTest

So the plug-in xml and the code needs to be removed.