[BSv5] Drop import of rtl/main

[chalin]

• As is mentioned in the changelog, this drops the BSv5-incompatible RTL support we had before. We drop the rtl/main import from the main SCSS file, but keep the rtl style files for now.
• Contributes to Reintroduce RTL support using BSv5 RTLCSS #1442, which we'll use to track progress of RTL-support reintroduction
• View diff by ignoring whitespace changes

[geriom]

Thanks!