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<?php

  function ccd_remove_image_size( $possible_sizes ) {
    unset( $possible_sizes['small'] );
    unset( $possible_sizes['medium'] );
    unset( $possible_sizes['large'] );
    return $possible_sizes;
  }
  
  add_filter( 'image_size_names_choose', 'ccd_remove_image_size' );

?>