Summary: We use JSS now and this remove support for SASS from our code base.

Test Plan: Tested with webpack.

Reviewers: michelle, nserrino, #engineering

Reviewed By: michelle, #engineering

Differential Revision: https://phab.corp.pixielabs.ai/D5594

GitOrigin-RevId: da506d65d569f845a2335432973b1b6d23e00058