Remove invalid Doxygen \ingroup tag

No other files in equityfx/ use \ingroup, and the group
'termstructures' doesn't exist in the Doxygen config.

Author: pandashark

ql/termstructures/volatility/equityfx/blackvariancesurfacefromsmilesections.hpp

@@ -36,7 +36,6 @@ namespace QuantLib {
         smile sections, one per tenor. It interpolates linearly in
         total variance between tenors for a given strike.
 
-        \ingroup termstructures
     */
     class BlackVarianceSurfaceFromSmileSections
         : public BlackVarianceTermStructure {