Test: remove test that will never run on master

This test was added in #23950 for backporting and review, but it is always skipped on master so
this commit deletes it.

rest-api-spec/src/main/resources/rest-api-spec/test/indices.create/20_warnings.yaml

@@ -1,28 +0,0 @@
----
-"Create index with deprecated settings":
-
-  - skip:
-      version: "all"
-      reason: removed in 6.0
-      features: "warnings"
-  - do:
-      indices.create:
-        index: test_index
-        body:
-          settings:
-            number_of_shards: 1
-            shadow_replicas: true
-            shared_filesystem: false
-          mappings:
-            type:
-              properties:
-                field:
-                  type: "string"
-                field2:
-                  type: "long"
-                  store : "no"
-      warnings:
-        - "[index.shadow_replicas] setting was deprecated in Elasticsearch and will be removed in a future release! See the breaking changes documentation for the next major version."
-        - "[index.shared_filesystem] setting was deprecated in Elasticsearch and will be removed in a future release! See the breaking changes documentation for the next major version."
-        - "The [string] field is deprecated, please use [text] or [keyword] instead on [field]"
-        - "Expected a boolean [true/false] for property [field2.store] but got [no]"