Remove fallbacks from theme var(...) calls (#14881)

This PR changes how we render `var(...)` calls for theme values,
removing the fallback values we were previously including.

```diff
  .text-white {
-   color: var(--color-white, #fff);
+   color: var(--color-white);
  }
```

We previously included the fallbacks only so you could see the value in
dev tools but this feels like a bad reason to bloat the CSS. I'd rather
just convince the Chrome team to surface this stuff better in dev tools
in the first place.

---------

Co-authored-by: Adam Wathan <4323180+adamwathan@users.noreply.github.com>

integrations/postcss/index.test.ts

@@ -404,7 +404,7 @@ test(
       candidate`hocus:underline`,
       css`
         .text-primary {
-          color: var(--color-primary, black);
+          color: var(--color-primary);
         }
       `,
     ])
@@ -446,7 +446,7 @@ test(
     await fs.expectFileToContain('project-a/dist/out.css', [
       css`
         .text-primary {
-          color: var(--color-primary, red);
+          color: var(--color-primary);
         }
       `,
     ])