Remove Request.Params#flatSettings leftover (#29676)

Relates to #29560

Author: javanna

client/rest-high-level/src/main/java/org/elasticsearch/client/Request.java

@@ -842,13 +842,6 @@ Params withLocal(boolean local) {
             return this;
         }
 
-        Params withFlatSettings(boolean flatSettings) {
-            if (flatSettings) {
-                return putParam("flat_settings", Boolean.TRUE.toString());
-            }
-            return this;
-        }
-
         Params withIncludeDefaults(boolean includeDefaults) {
             if (includeDefaults) {
                 return putParam("include_defaults", Boolean.TRUE.toString());