delete `operation_threading` from the rest specs (#27940)

rest-api-spec/src/main/resources/rest-api-spec/api/indices.forcemerge.json

@@ -38,9 +38,6 @@
           "type" : "boolean",
           "description" : "Specify whether the operation should only expunge deleted documents"
         },
-        "operation_threading": {
-          "description" : "TODO: ?"
-        },
         "wait_for_merge": {
           "type" : "boolean",
           "description" : "Specify whether the request should block until the merge process is finished (default: true)"

rest-api-spec/src/main/resources/rest-api-spec/api/indices.segments.json

@@ -26,9 +26,6 @@
             "default" : "open",
             "description" : "Whether to expand wildcard expression to concrete indices that are open, closed or both."
         },
-        "operation_threading": {
-          "description" : "TODO: ?"
-        },
         "verbose": {
           "type": "boolean",
           "description": "Includes detailed memory usage by Lucene.",

rest-api-spec/src/main/resources/rest-api-spec/api/indices.shard_stores.json

@@ -30,9 +30,6 @@
             "options" : ["open","closed","none","all"],
             "default" : "open",
             "description" : "Whether to expand wildcard expression to concrete indices that are open, closed or both."
-        },
-        "operation_threading": {
-          "description" : "TODO: ?"
         }
       }
     },

rest-api-spec/src/main/resources/rest-api-spec/api/indices.validate_query.json

@@ -34,9 +34,6 @@
             "default" : "open",
             "description" : "Whether to expand wildcard expression to concrete indices that are open, closed or both."
         },
-        "operation_threading": {
-          "description" : "TODO: ?"
-        },
         "q": {
           "type" : "string",
           "description" : "Query in the Lucene query string syntax"