Remove header from yaml test

Author: ioanatia

x-pack/plugin/ent-search/qa/rest/src/yamlRestTest/resources/rest-api-spec/test/entsearch/rules/81_query_rules_retriever_no_rulesets.yml

@@ -13,9 +13,6 @@ setup:
   - skip:
       features: [ headers ]
   - do:
-      headers:
-        # Force JSON content type so that we use a parser that interprets the floating-point score as a double
-        Content-Type: application/json
       search:
         index: test-index1
         body: