[4.0] Remove Cassiopeias "_card.scss" override

[Bakual]

Since PR #30734 is already RTC this is a followup to remove the Cassiopeia Override for Bootstrap cards.
That file only contains two additional card specific classes: "card-grey" and "card-inverse".

A very similar effect like those two classes can be achieved by using the standard Bootstrap colour classes (see https://getbootstrap.com/docs/4.5/utilities/colors/):

Summary of Changes

Deleting the whole file and the reference in the template.

Testing Instructions

• Apply PR
• run "npm run build.css" to regenerate the template CSS file
• Compare module output and other appearance of card layouts (are there any?)
• Add "card-grey" and "card-inverse" to a module class and assign that module to a position with actually uses the card chrome (eg top-b or sidebars).

Actual result BEFORE applying this Pull Request

The module has a slight darker greyish background with "card-grey" and text becomes white with "card-inverse"

Expected result AFTER applying this Pull Request

The classes have no effect anymore.

Documentation Changes Required

None

[wilsonge]

Thanks!