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 <[email protected]>

packages/@tailwindcss-postcss/src/__snapshots__/index.test.ts.snap

@@ -541,12 +541,12 @@ exports[`\`@import 'tailwindcss'\` is replaced with the generated CSS 1`] = `
 
 @layer utilities {
   .text-2xl {
-    font-size: var(--font-size-2xl, 1.5rem);
-    line-height: var(--tw-leading, var(--font-size-2xl--line-height, 2rem));
+    font-size: var(--font-size-2xl);
+    line-height: var(--tw-leading, var(--font-size-2xl--line-height));
   }
 
   .text-black\\/50 {
-    color: color-mix(in oklch, var(--color-black, #000) 50%, transparent);
+    color: color-mix(in oklch, var(--color-black) 50%, transparent);
   }
 
   .underline {

packages/@tailwindcss-postcss/src/index.test.ts

@@ -100,7 +100,7 @@ test('@apply can be used without emitting the theme in the CSS file', async () =
 
   expect(result.css.trim()).toMatchInlineSnapshot(`
     ".foo {
-      color: var(--color-red-500, oklch(.637 .237 25.331));
+      color: var(--color-red-500);
     }"
   `)
 })