Remove Bootstrap 4's Cards components from dashboard/todos

With the Bootstrap migration from 3 to 4 (gitlab-foss#45185 (closed)), we introduced app/assets/stylesheets/bootstrap_migration.scss for the rapid integration in gitlab-foss!18232 (merged). For going forward to Bootstrap 5 (&4265 (closed)) or whatever we will use in the future, we should remove migration styles.

With this tackle on ~"technical debt", .card-without-margin selector would be removed from app/assets/stylesheets/bootstrap_migration.scss.