[Super Cache] Remove the "wp-content is writeable" notice, as updating the setting requires it.

### Impacted plugin

Super Cache

### Quick summary

When the plugin is first installed it requires write access to the wp-content directory. Once that access is granted the configuration file and advanced-cache.php file are created.
However, when the settings page loads, it shows a warning, saying "Warning! [path to web server]/wp-content is writeable!" with instructions on how to fix it.

This was fine in the past when wp-cache-config.php was updated "in place" but now it's updated in a temporary file and renamed when settings are changed. To do that requires write access to the directory.

The notice can be dismissed, which is probably what the vast majority of people do without looking at it.

As we need a writeable directory, we should remove the notice informing the user that wp-content is writeable.

### Steps to reproduce

Install the plugin.
Go to the settings page.
Observe the notice if wp-content is writeable.
Make it read-only
Configuration can't be updated.

### A clear and concise description of what you expected to happen.

When I made wp-content read-only I expected that I could update the plugin configuration.

### What actually happened

With wp-content read-only I couldn't update the plugin configuration. It silently failed to update (which is another bug I guess)

### Impact

One

### Available workarounds?

No and the platform is unusable

### Platform (Simple and/or Atomic)

Self-hosted

### Logs or notes

_No response_

Start by locating the settings-page warning and the configuration update path described in the issue, then reproduce the behavior with wp-content writable and read-only. Done should include removing the misleading writable-directory notice and verifying whether configuration updates remain functional under the required permissions.

Written by the indexing model from the issue text.