How can I remove the ::after CSS selector which is automaticaly appended to the_content()?

It has nothing to do with the_content().

::after is a pseudo element that’s added with CSS. Note that the ::after element in the dev tools is only there to help development. It does not exist in the HTML source.

So your theme’s CSS must be adding it to whatever element is wrapping the_content() in your template. You can remove it by setting content: none; on the selector for it. For example, if the element that’s wrapping your content is <div class="entry-content"> then the CSS to remove it could be:

.entry-content::after {
    content: none;
}

The rules of specificity still apply though, so whatever selector you use to remove it needs to be more specific than the one that added it.

Also keep in mind that your theme likely does this for a reason. Most commonly elements like this are used to clear floats and prevent float-aligned content, like images, from breaking outside the parent element.