Avoid using the grant-API-key-API in yml test (#105402)

The grant API key API is disabled in serverless.
Rather than mute the Query API Key yml test (21_query_with_aggs),
which uses the grant API key API, for the serverless build,
I've here modified the yml test to not invoke the grant API Key API.
Querying for granted API keys is already covered
(e.g. ApiKeyAggsIT#testFilterAggs).

Relates #104895

x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/api_key/21_query_with_aggs.yml

@@ -107,28 +107,6 @@ teardown:
   - transform_and_set: { login_creds: "#base64EncodeCredentials(id,api_key)" }
   - match: { encoded: $login_creds }
 
-  - do:
-      headers:
-        Authorization: "Basic YXBpX2tleV9tYW5hZ2VyOngtcGFjay10ZXN0LXBhc3N3b3Jk" # api_key_manager
-      security.grant_api_key:
-        body:  >
-          {
-            "api_key": {
-              "name": "user-api-key-grant",
-              "expiration": "100d"
-            },
-            "grant_type": "password",
-            "username": "api_key_user",
-            "password": "x-pack-test-password"
-          }
-  - match: { name: "user-api-key-grant" }
-  - is_true: id
-  - is_true: api_key
-  - is_true: expiration
-  - set: { id: api_key_id }
-  - transform_and_set: { login_creds: "#base64EncodeCredentials(id,api_key)" }
-  - match: { encoded: $login_creds }
-
   # match_all with composite aggregation
   - do:
       headers:
@@ -151,16 +129,13 @@ teardown:
               }
             }
           }
-  - match: { total: 3 }
+  - match: { total: 2 }
   - match: { count: 0 }
   - length: { api_keys: 0}
-  - length: { aggregations.my_buckets.buckets: 3}
+  - length: { aggregations.my_buckets.buckets: 2}
 
   - match: { aggregations.my_buckets.buckets.0.key.key_name: "manager-api-key" }
   - match: { aggregations.my_buckets.buckets.0.doc_count: 1 }
 
   - match: { aggregations.my_buckets.buckets.1.key.key_name: "user-api-key" }
   - match: { aggregations.my_buckets.buckets.1.doc_count: 1 }
-
-  - match: { aggregations.my_buckets.buckets.2.key.key_name: "user-api-key-grant" }
-  - match: { aggregations.my_buckets.buckets.2.doc_count: 1 }