Twenty Twenty-One: fix leftover rtlcss call in build script

[yoavf]

The build:rtl script for Twenty Twenty-One has style-dark-mode.css param. This is probably a leftover as that file is now in assets/css/ and has its own rtl build script build:dark-rtl.

While rtlcss doesn't output an error, you can see that only style.css is processed when calling npm run build:rtl

> twentytwentyone@1.1.0 build:rtl ~/wordpress-develop/src/wp-content/themes/twentytwentyone
> rtlcss style.css style-rtl.css style-dark-mode.css style-dark-mode-rtl.css

Saving: ~/wordpress-develop/src/wp-> content/themes/twentytwentyone/style-rtl.css

[SergeyBiryukov]

In 49960:

Twenty Twenty-One: Remove leftover file from build:rtl task.

The style-dark-mode.css file is now in assets/css/ and has its own build:dark-rtl task.

Props yoavf.
Fixes #52293.