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/cli/index.test.ts

@@ -167,7 +167,7 @@ describe.each([
         candidate`hocus:underline`,
         css`
           .text-primary {
-            color: var(--color-primary, black);
+            color: var(--color-primary);
           }
         `,
       ])
@@ -207,7 +207,7 @@ describe.each([
       await fs.expectFileToContain('project-a/dist/out.css', [
         css`
           .text-primary {
-            color: var(--color-primary, red);
+            color: var(--color-primary);
           }
         `,
       ])

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);
         }
       `,
     ])

integrations/vite/index.test.ts

@@ -334,7 +334,7 @@ for (let transformer of ['postcss', 'lightningcss']) {
           candidate`flex`,
           css`
             .text-primary {
-              color: var(--color-primary, black);
+              color: var(--color-primary);
             }
           `,
         ])
@@ -356,7 +356,7 @@ for (let transformer of ['postcss', 'lightningcss']) {
 
           expect(styles).toContain(css`
             .text-primary {
-              color: var(--color-primary, red);
+              color: var(--color-primary);
             }
           `)
         })