PHP basics help in WP context – remove a class/function?

You’ll need to see, if the plugin author has used apply_filters() in the function that renders the metabox fields. If there is one, then you can attach your filtering function to that hook and remove unnecessary fields from the metabox.

The code below is pseudo-code and for example use only.

Inside the class there could be a method (i.e. function) along these lines.

public function render_metabox_fields() {
  $fields = apply_filters( 'filter_available_fields', $this->get_file_fields() );
  // some loop code to actually render the fields...  
}

The rendering might also happen in some other part of the codebase – who knows, as with all 3rd party plugins/themes.

You could then do this in your theme’s functions.php file,

add_filter('filter_available_fields', 'my_descriptive_function_name');
function my_descriptive_function_name( $fields ) {
  // loop available fields which the apply_filter function provides as a parameter for our function
  foreach ($fields as $index => $field) {
    // target only certain items with if statement
    if ( 'video' === $field['setting'] ) {
      // do something with matched items
      unset($fields[$index]);
    }
  }
  // return variable to the apply_filters function so that the original code can continue its operations
  return $fields;
}

But, if there are no filtering hooks to attach custom code to, then you’re probably out of luck. You better check from the plugin author or documentation.