Make default_operation optional in netconf_config module (#46333)

Fixes #46257

*  As per netconf rfc default-operation value is optional,
   hence removing the default value for default_operation
   option.

(cherry picked from commit 5e6eb921ae)

* Update changelog

changelogs/fragments/netconf_config_default_operation.yaml

@@ -0,0 +1,3 @@
+---
+minor_changes:
+- netconf_config - Make default_operation optional in netconf_config module (https://github.com/ansible/ansible/pull/46333)

lib/ansible/modules/network/netconf/netconf_config.py

@@ -71,7 +71,6 @@ options:
       option completely replaces the configuration in the C(target) datastore. If the value is none the C(target)
       datastore is unaffected by the configuration in the config option, unless and until the incoming configuration
       data uses the C(operation) operation to request a different operation.
-    default: merge
     choices: ['merge', 'replace', 'none']
     version_added: "2.7"
   confirm:
@@ -231,7 +230,7 @@ def main():
         source_datastore=dict(aliases=['source']),
         format=dict(choices=['xml', 'text'], default='xml'),
         lock=dict(choices=['never', 'always', 'if-supported'], default='always'),
-        default_operation=dict(choices=['merge', 'replace', 'none'], default='merge'),
+        default_operation=dict(choices=['merge', 'replace', 'none']),
         confirm=dict(type='int', default=0),
         confirm_commit=dict(type='bool', default=False),
         error_option=dict(choices=['stop-on-error', 'continue-on-error', 'rollback-on-error'], default='stop-on-error'),