Remove background from nested tables

Created by: MartijnCuppens

This was once added here https://github.com/twbs/bootstrap/commit/9376a7c221a64c2bf508d02ea2ccd85748d10fcc, but it's not clear why this was added. Anyway, this causes the issue mentioned in #27311 (closed).

Fixes https://github.com/twbs/bootstrap/issues/27311