Get transaction API not working

[jessy-p]

Describe the bug

The Get Transaction API /payments/transactions/<transaction_id> does not work.

To Reproduce

1. Set up a test site, enable REST API keys to use the WC REST APIs.
2. Place a few orders.
3. Replace test_site, customer_key, customer_secret in below URL, and see that the transactions are retrieved.

curl -X GET "https://<test_site.com>/wp-json/wc/v3/payments/transactions?page=1&pagesize=25&sort=date&direction=desc" \
  -u <customer_key>:<customer_secret> \
  -H "Content-Type: application/json"

4. Similarly replace the values below, and use a transaction_id retrieved from previous API. Observe no result retrieved.

curl -X GET "https://<test_site.com>/wp-json/wc/v3/payments/transactions/<transaction_id>" \
  -u <customer_key>:<customer_secret> \
  -H "Content-Type: application/json"

Actual behavior

No result retrieved when trying to fetch a transaction detail using the get transaction API

Screenshots

N/A

Expected behavior

Transaction detail should be retrieved.

Additional context

Might be related
PHP Warning: call_user_func_array() expects parameter 1 to be a valid callback, class 'WC_Payments_API_Client' does not have a method 'get_transactions' in /Users/jessyp/code/woocommerce-payments/includes/admin/class-wc-payments-rest-controller.php on line 50

[jessy-p]

• Not required as part of WCPay Extensibility APIs anymore.
• The code is present to expose a GET transaction endpoint, but it does not work, so we may remove the redundant code as the front-end does not need this API.

[zmaglica]

Addressed in #10131