[nfc] Remove unneeded void cast of clangDC.

This is just a simple cleanup.

Author: zoecarver

lib/AST/ASTMangler.cpp

@@ -1816,7 +1816,6 @@ ASTMangler::getSpecialManglingContext(const ValueDecl *decl,
         if (isa<clang::NamespaceDecl>(clangDC)) return None;
         assert(clangDC->getRedeclContext()->isTranslationUnit() &&
                "non-top-level Clang types not supported yet");
-        (void)clangDC;
         return ASTMangler::ObjCContext;
       }
     }