All rules that uses custom CSS properties is removed

Input:

```twig
{% htmlcompress %}
<style>
.foo {
    --a: 1em;
    padding: var(--a);
    color: blue;
}
</style>
{% endhtmlcompress %}
```

Expected output:

```html
<style>.foo {--a: 1em;padding: var(--a);color: blue;}</style>
```

Actual output:

```html
<style>.foo {--a: 1em;color: blue;}</style>
```