How to dequeue theme’s RTL stylesheet?

RTL stylesheet is not loaded with wp_enqueue_style() so it wouldn’t appear in global $wp_styles variable. WordPress loads it with get_locale_stylesheet() function. You can remove loading locale scripts completely:

function abort_loading_rtl_stylesheet() {
    remove_action( 'wp_head', 'locale_stylesheet' );
}
add_action( 'init', 'abort_loading_rtl_stylesheet' );

or filter its output to load your custom rtl stylesheet:

function override_default_rtl_styles(){
    return 'path/to/custom/rtl.css';
}

add_filter( 'locale_stylesheet_uri', 'override_default_rtl_styles', 1000 );

I hope that’s what you’re looking for, cheers!