Xor8: remove previous serialization methods

[vprus]

The PR #47 added serialization method to the Xor family of filters.

However, Xor8 already had two serialization methods, and they used a different format (without encoded length of the fingerprint array). Having two serialization methods with a different format is just asking for trouble, so this PR
removes the historic method. Test appear to still work.

[lemire]

@vprus Could you just deprecate it instead ? So we don't break existing code ?

[vprus]

I can, but is there evidence this code is used? Xor8 iself is documented as experimental.

[lemire]

I can, but is there evidence this code is used? Xor8 iself is documented as experimental.

None as far as I can tell, but it is arguably easier to deprecate than to delete. https://www.baeldung.com/java-deprecated

I have just recently added a note to the effect that it is experimental. The implementation is ok, but it is certainly not battle tested.

[vprus]

Ok, updated to use @deprecated.

[lemire]

Yeah. So this is super safe now. We can delete it later after a long time.