Better handle many (MANY) duplicate refs

In large designs with thousands of duplicate elements (e.g. power
symbols), we need to be smart about vector management.  Removing all but
the first few items should be batched and not element at a time

Ref: KSC-885

eeschema/sch_reference_list.cpp

@@ -634,8 +634,7 @@ void SCH_REFERENCE_LIST::Annotate( bool aUseSheetNum, int aSheetIntervalId, int
     }
 
     // Remove aAdditionalRefs references
-    for( size_t i = originalSize; i < ( aAdditionalRefs.GetCount() + originalSize ); i++ )
-        RemoveItem( originalSize );
+    m_flatList.erase( m_flatList.begin() + originalSize, m_flatList.end() );
 
     wxASSERT( originalSize == GetCount() ); // Make sure we didn't make a mistake
 }