[ClangImporter] Remove unused variable. NFC. (#4426)

Author: jrose-apple

lib/ClangImporter/ImportDecl.cpp

@@ -5027,7 +5027,6 @@ namespace {
                              const clang::ObjCProtocolList &clangProtocols,
                              SmallVectorImpl<TypeLoc> &inheritedTypes) {
       SmallVector<ProtocolDecl *, 4> protocols;
-      llvm::SmallPtrSet<ProtocolDecl *, 4> knownProtocols;
       if (auto nominal = dyn_cast<NominalTypeDecl>(decl))
         nominal->getImplicitProtocols(protocols);