How to hide "posts in category" text from catogry filter page?

Try adding this to the your custom css after any other code that may be there

main.Main.Main--blog-list .BlogList-filter {
    display: none;
}

If it doesn't work please provide a link to the site in question. 

Thanks it works!!! Thank you so much!

But it does appear for a few seconds before disappearing, and actually all my code does not work immediately, only after few seconds or after refreshing page. Do you have any idea what could be causing this? Enabling or disabling ajax loading has no effect.

It's not possible to diagnose any problem without at least seeing your site and, ideally, having administrator access. 

If you share a link here we may be able to give you some tips. 

If you want a proper diagnosis and (if possible) fix  it would require Admin access to your site and would be a paying job.  Contact me if you require this.