More warning fixes iG-API on Windows.

modules/gapi/src/backends/common/serialization.cpp

@@ -393,10 +393,9 @@ IOStream& operator<< (IOStream& os, const cv::GArrayDesc &) {return os;}
 IIStream& operator>> (IIStream& is,       cv::GArrayDesc &) {return is;}
 
 #if !defined(GAPI_STANDALONE)
-IOStream& operator<< (IOStream& os, const cv::UMat &)
+IOStream& operator<< (IOStream&, const cv::UMat &)
 {
     GAPI_Error("Serialization: Unsupported << for UMat");
-    return os;
 }
 IIStream& operator >> (IIStream&, cv::UMat &)
 {

modules/gapi/src/compiler/gcompiler.cpp

@@ -341,7 +341,6 @@ void cv::gimpl::GCompiler::validateInputMeta()
         default:
             GAPI_Error("InternalError");
         }
-        return false; // should never happen
     };
 
     GAPI_LOG_DEBUG(nullptr, "Total count: " << m_metas.size());