Remove unecessary sass import

CollapsibleNav vars now provided via theme import
see opensearch-project/oui#968

Signed-off-by: Josh Romero <rmerqg@amazon.com>

src/core/public/styles/_base.scss

@@ -13,9 +13,6 @@
 @import "@elastic/charts/dist/theme";
 @import "@elastic/eui/src/themes/charts/theme";
 
-// Grab some nav-specific EUI vars
-@import "@elastic/eui/src/components/collapsible_nav/variables";
-
 // Application Layout
 $euiCollapsibleNavWidth: $euiSize * 20;