fix: remove unused code in system-setting.yaml (#6411)

#### What type of PR is this?

/kind cleanup
/area core

#### What this PR does / why we need it:

移除在 https://github.com/halo-dev/halo/pull/6403 中误推送的代码。

#### Does this PR introduce a user-facing change?


```release-note
None
```

application/src/main/resources/extensions/system-setting.yaml

@@ -193,7 +193,3 @@ spec:
           label: "页脚"
           name: footer
           help: "注入代码到所有页面的页脚部分"
-        - $formkit: pay
-          productId: xxx
-          help: xxx
-          validation: required