Remove ClusterStateSerializationTests

These tests do not seem valuable anymore, now that multi-project has
been merged some time ago. Additionally, creating objects for a cluster
state this way is tricky, as those objects might get new
features/fields, which would break serialization.

Closes #130872

Author: nielsbauman

muted-tests.yml

@@ -558,9 +558,6 @@ tests:
 - class: org.elasticsearch.xpack.slm.SLMFileSettingsIT
   method: testSettingsApplied
   issue: https://github.com/elastic/elasticsearch/issues/130853
-- class: org.elasticsearch.cluster.ClusterStateSerializationTests
-  method: testSerializationPreMultiProject
-  issue: https://github.com/elastic/elasticsearch/issues/130872
 
 # Examples:
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