Remove deprecated shopify app scaffold extension command

The `shopify app scaffold extension` command was a hidden alias for
`shopify app generate extension`, deprecated long ago. The file was
already unregistered from the CLI command map (`pnpm shopify app:scaffold:extension`
returns 'command not found'), so this change just deletes the orphaned source.

Author: alfonso-noriega

.changeset/remove-app-scaffold-extension-command.md

@@ -0,0 +1,5 @@
+---
+'@shopify/app': major
+---
+
+Remove the deprecated `shopify app scaffold extension` command. Use `shopify app generate extension` instead. (The command was already unregistered from the CLI command map but the file remained on disk; this change deletes the orphaned source file.)