CRM_Utils_System_WordPress - Drop unused variable

CRM_Utils_System_WordPress - Drop unused variable

This is an off-shoot from #24911.

Before
------

`WordPress` implements the method `mapConfigToSSL()`, but ti doesn't do anything.

To see this, you can search for 'base_url' across the entire-source of a Civi-WP installation.
Specifically, I used

```
grep -lri 'global.*base_url'
```

The references appeared unrelated -- some references to Drupal/Backdrop adapters, and
some references to minified JS files. But I couldn't see anything that would in PHP on WP.

After
-----

Don't bother implementing the method. Might as well inherit the base method (which is empty).