Fix sourcemaps for internal client boundaries

We used to manually insert `use client` where SWC strips it.
This caused sourcemaps to be off by one line.

The underlying issue in SWC is fixed so we can remove the workaround
and fix sourcemaps for internal files with a `use client` directive.

packages/next/taskfile-swc.js

@@ -128,17 +128,6 @@ module.exports = function (task) {
       const output = yield transform(source, options)
       const ext = path.extname(file.base)
 
-      // Make sure the output content keeps the `"use client"` directive.
-      // TODO: Remove this once SWC fixes the issue.
-      if (/^['"]use client['"]/.test(source)) {
-        output.code =
-          '"use client";\n' +
-          output.code
-            .split('\n')
-            .map((l) => (/^['"]use client['"]/.test(l) ? '' : l))
-            .join('\n')
-      }
-
       // Replace `.ts|.tsx` with `.js` in files with an extension
       if (ext) {
         const extRegex = new RegExp(ext.replace('.', '\\.') + '$', 'i')