Remove or hide featured image from post page?

You can do this by overriding the pluggable function cpotheme_postpage_image().

A pluggable function is such a function which you can override in your child theme. A pluggable function is always enclosed inside a condition like

if(!function_exists('pluggable_function_name')) {
  //declare the function here
  function pluggable_function_name() {
    //function body here
  } 
}

So if you write a function with the same name in your child theme’s functions.php file like this:

function pluggable_function_name() {
  //function body here
}

WordPress will not execute the function in the parent theme and your function will be taken into precedence.

In your case the plugabble function is cpotheme_postpage_image() which is originally written as

function cpotheme_postpage_image(){
		if(has_post_thumbnail()){
			if(!is_singular('post')){
				echo '<a href="'.get_permalink(get_the_ID()).'" title="'.sprintf(esc_attr__('Go to %s', 'allegiant'), the_title_attribute('echo=0')).'" rel="bookmark">';
				the_post_thumbnail('portfolio');
				echo '</a>';
			}else{
				the_post_thumbnail();
			}
		}
	}

which you need to override in your child theme’s functions.php file like this:

function cpotheme_postpage_image(){
		if(has_post_thumbnail()){
			if(!is_singular('post')){
				echo '<a href="'.get_permalink(get_the_ID()).'" title="'.sprintf(esc_attr__('Go to %s', 'allegiant'), the_title_attribute('echo=0')).'" rel="bookmark">';
				the_post_thumbnail('portfolio');
				echo '</a>';
			}else{
				echo '';
			}
		}
	}

meaning if this is single post, the function outputs empty string (echo '';) and does not print the_post_thumbail();, i.e. the featured image.

Let me know if this helps.