Revert "enable shadows"

This reverts commit b2f3600a407566d8ab4973f0a18b4b24fd2cc24c.

packages/core/scss/bootstrap/_variables.scss

@@ -25,13 +25,6 @@ $warning-border-subtle:       mix(#fff, $warning, 70%) !default;
 $danger-border-subtle:        mix(#fff, $danger, 70%) !default;
 
 
-// Options
-//
-// Quickly modify global styling by enabling or disabling optional features.
-
-$enable-shadows: true;
-
-
 // Links
 //
 // Style anchor elements.