daemon: override keyword is not supported by older gcc

daemon/m4/ax_cxx_compile_stdcxx_11.m4

@@ -43,13 +43,6 @@ m4_define([_AX_CXX_COMPILE_STDCXX_11_testbody], [[
       static_assert(sizeof(int) <= sizeof(T), "not big enough");
     };
 
-    struct Base {
-    virtual void f() {}
-    };
-    struct Child : public Base {
-    virtual void f() override {}
-    };
-
     typedef check<check<bool>> right_angle_brackets;
 
     int a;