Remove reference to dropped EIS API (#126422)

Co-authored-by: Quentin Pradet <quentin.pradet@elastic.co>

rest-api-spec/src/main/resources/rest-api-spec/api/inference.put_eis.json

@@ -1,35 +0,0 @@
-{
-    "inference.put_eis": {
-      "documentation": {
-        "url": "https://www.elastic.co/guide/en/elasticsearch/reference/current/infer-service-elastic.html",
-        "description": "Configure an inference endpoint that uses the Elastic Inference Service (EIS)"
-      },
-      "stability": "stable",
-      "visibility": "public",
-      "headers": {
-        "accept": ["application/json"],
-        "content_type": ["application/json"]
-      },
-      "url": {
-        "paths": [
-          {
-            "path": "/_inference/{task_type}/{eis_inference_id}",
-            "methods": ["PUT"],
-            "parts": {
-              "task_type": {
-                "type": "string",
-                "description": "The task type"
-              },
-              "eis_inference_id": {
-                "type": "string",
-                "description": "The inference ID"
-              }
-            }
-          }
-        ]
-      },
-      "body": {
-        "description": "The inference endpoint's task and service settings"
-      }
-    }
-  }