Revert "Fix namespace for SIFT"

This reverts commit dd454f77.

src/descriptors/sift/descriptor_sift.h

@@ -48,7 +48,7 @@ inline void DescriptorSIFT::defineDescriptor(const ParamsBasePtr _params)
 {
     DescriptorParamsSIFTPtr params_ptr = std::static_pointer_cast<DescriptorParamsSIFT>(_params);
 
-    descriptor_ = cv::xfeatures2d::SIFT::create(params_ptr->nfeatures,
+    descriptor_ = cv::SIFT::create(params_ptr->nfeatures,
     		params_ptr->nOctaveLayers,
 			params_ptr->contrastThreshold,
 			params_ptr->edgeThreshold,

src/detectors/sift/detector_sift.h

@@ -53,7 +53,7 @@ inline void DetectorSIFT::defineDetector(const ParamsBasePtr _params)
 {
     DetectorParamsSIFTPtr params_ptr = std::static_pointer_cast<DetectorParamsSIFT>(_params);
 
-    detector_ = cv::xfeatures2d::SIFT::create(params_ptr->nfeatures,
+    detector_ = cv::SIFT::create(params_ptr->nfeatures,
     		params_ptr->nOctaveLayers,
 			params_ptr->contrastThreshold,
 			params_ptr->edgeThreshold,