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