Removed unused variable [NFC]

The compiler warned:
../tools/clang/lib/StaticAnalyzer/Core/RetainSummaryManager.cpp:329:19: error: unused variable 'MD' [-Werror,-Wunused-variable]
  if (const auto *MD = dyn_cast<CXXMethodDecl>(FD)) {
                  ^
1 error generated.

llvm-svn: 340524

clang/lib/StaticAnalyzer/Core/RetainSummaryManager.cpp

@@ -326,7 +326,7 @@ RetainSummaryManager::generateSummary(const FunctionDecl *FD,
     }
   }
 
-  if (const auto *MD = dyn_cast<CXXMethodDecl>(FD)) {
+  if (dyn_cast<CXXMethodDecl>(FD)) {
 
     // Stop tracking arguments passed to C++ methods, as those might be
     // wrapping smart pointers.