Revert to default font family

Why these changes are being introduced:

TIMDEX UI had been overriding the default font family we use
in the style guide (Roboto instead of Helvetica Neue). Darcy has
requested that we use the default font family.

Relevant ticket(s):

* [GDT-215](https://mitlibraries.atlassian.net/browse/GDT-215)
* [GDT-239](https://mitlibraries.atlassian.net/browse/GDT-239)

How this addresses that need:

This removes the font overrides.

Side effects of this change:

TIMDEX UI has a few other typography overrides. It's not clear to
me if those are still relevant, but Darcy will let us know during
UX review.

Author: jazairi

app/assets/stylesheets/application.css.scss

@@ -3,7 +3,6 @@
 */
 
 @import "libraries-main";
-@import "fonts";
 @import "partials/_variables";
 @import "partials/_alerts";
 @import "partials/_filters";
@@ -18,4 +17,3 @@
 @import "partials/_shared";
 @import "partials/_results";
 @import "partials/_typography";
-@import "timdexui";

app/assets/stylesheets/fonts.scss

@@ -2,9 +2,6 @@
 // #TYPOGRAPHY
 // ----------------------------
 
-// Font stack
-$base-font: 'Roboto', 'Helvetica Neue', Helvetica, Arial, sans-serif;
-
 // Font sizes, weights, and line heights
 // 1 rem is 10 px...
 $fs-base:      1.6rem; // Pentagram originally specified 1.4