Remove filter callback: method in class

Note that to remove a filter callback from the woocommerce_email_content filter, you first need to have it registered via:

add_filter( 'woocommerce_email_content', ...some-custom-callback... );

but it looks like you’re trying to remove the inline styles from the message.

I guess you need the text/html content type, so try instead to override the styles with (untested):

add_filter( 'woocommerce_email_styles', '__return_null' );