How to edit the styles for the table of pages/post listed in the admin when you click on pages/posts?

I guess you are in edit.php so you can add style only for this admin page.

function enqueue_my_scripts($hook) {
if ( 'edit.php' != $hook ) {
    return;
    }
  //our script to remove fixed table class
 echo '<script>';
 echo '$( ".wp-list-table" ).removeClass( "fixed" )';
 echo '</script>';

}
add_action( 'admin_enqueue_scripts', 'enqueue_my_scripts' );

The explanation is basic first we check if we are in edit page then execute the remove class.