Partially revert "categorized renderer: also setup symbol render context for hashed symbols"

This partially reverts commit c7c5244.

Causes a race condition which results in random crashes while
using the categorised symbol renderer.

The intent behind the original commit is unknown

Author: nyalldawson

src/core/symbology-ng/qgscategorizedsymbolrendererv2.cpp

@@ -445,12 +445,6 @@ void QgsCategorizedSymbolRendererV2::startRender( QgsRenderContext& context, con
       mTempSymbols[ cat.symbol()] = tempSymbol;
     }
   }
-
-  Q_FOREACH ( QgsSymbolV2 *symbol, mSymbolHash.values() )
-  {
-    symbol->startRender( context, &fields );
-  }
-
   return;
 }
 
@@ -461,11 +455,6 @@ void QgsCategorizedSymbolRendererV2::stopRender( QgsRenderContext& context )
     cat.symbol()->stopRender( context );
   }
 
-  Q_FOREACH ( QgsSymbolV2 *symbol, mSymbolHash.values() )
-  {
-    symbol->stopRender( context );
-  }
-
   // cleanup mTempSymbols
   QHash<QgsSymbolV2*, QgsSymbolV2*>::const_iterator it2 = mTempSymbols.constBegin();
   for ( ; it2 != mTempSymbols.constEnd(); ++it2 )