Hide “Featured Image” in post – Megatron theme

Hi,
I want to hide the featured image in all posts or single post.
I have Megatron theme.

I tried to do everything I saw on Google but I can′t solve this.

I tested to add in custom CSS all these and more:
.single-post .attachment-post-thumbnail {
display: none;
}
.attachment-post-thumbnail {
display:none;
}
#top.single-post .big-preview.single-big {
display: none;
}
.single-portfolio .page-thumb {
display: none;
}
.single .big-preview.single-big {
display: none;
}
.single .post-image {display: none;}

Also, I did any change with PHP editor but nothing.

I don′t know programming I only did changes that I see on Google.

Help me please.

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