Don't test default serialize max_depth

Don't test default serialize max_depth

The default depth is large enough to cause stack overflows in
msan builds, though apparently only on old clang versions. Avoiding
a stack overflow there requires making the depth *much* smaller,
less than 1000. As I don't think that's reasonable for all the other
setups where 4k works fine, I'm just dropping this part of the test.