Importing regression fix – support old exported Playground ZIPs

Closes #1543

Playground exports before changes from the Boot Protocol (#1398)
contained platform-level WordPress plugins and mu-plugins. The importer,
then, removed them and replaced them with the latest version freshly sourced
from the /wordpress directory.

However, after the Boot Protocol changes, Playground no longer includes any
platform-level plugins in the /wordpress directory. This means that
things like SQLite database integration were being removed from the
imported ZIP bundle but they were not restored.

This PR removes this "remove & restore" mechanism. If the ZIP comes with
its own SQLite database integration, that one will be used. If it
doesn't, then Playground will fall back to the platform-level plugin.

 ## Testing instructions

1. Export Playground as zip and import it again. Confirm this worked.
2. Run [this Blueprint that imports an older ZIP bundle](https://playground.wordpress.net/builder/builder.html#{%22landingPage%22:%22/%22,%22preferredVersions%22:{%22php%22:%228.0%22,%22wp%22:%22latest%22},%22phpExtensionBundles%22:[%22kitchen-sink%22],%22features%22:{%22networking%22:true},%22steps%22:[{%22step%22:%22importWordPressFiles%22,%22wordPressFilesZip%22:{%22resource%22:%22url%22,%22url%22:%22https://kybernaut.cz/blueprints/pay-for-payment-playground.zip%22}},{%22step%22:%22login%22,%22username%22:%22admin%22,%22password%22:%22password%22},{%22step%22:%22installPlugin%22,%22pluginZipFile%22:{%22resource%22:%22wordpress.org/plugins%22,%22slug%22:%22woocommerce-pay-for-payment%22},%22options%22:{%22activate%22:true}}]}) and confirm it works

CC @vyskoczilova

packages/playground/blueprints/src/lib/steps/import-wordpress-files.ts

@@ -1,8 +1,7 @@
 import { StepHandler } from '.';
 import { unzip } from './unzip';
-import { dirname, joinPaths, phpVar } from '@php-wasm/util';
+import { joinPaths, phpVar } from '@php-wasm/util';
 import { UniversalPHP } from '@php-wasm/universal';
-import { wpContentFilesExcludedFromExport } from '../utils/wp-content-files-excluded-from-exports';
 import { defineSiteUrl } from './define-site-url';
 
 /**
@@ -59,28 +58,6 @@ export const importWordPressFiles: StepHandler<
 	});
 	importPath = joinPaths(importPath, pathInZip);
 
-	// Carry over any Playground-related files, such as the
-	// SQLite database plugin, from the current wp-content
-	// into the one that's about to be imported
-	const importedWpContentPath = joinPaths(importPath, 'wp-content');
-	const wpContentPath = joinPaths(documentRoot, 'wp-content');
-	for (const relativePath of wpContentFilesExcludedFromExport) {
-		// Remove any paths that were supposed to be excluded from the export
-		// but maybe weren't
-		const excludedImportPath = joinPaths(
-			importedWpContentPath,
-			relativePath
-		);
-		await removePath(playground, excludedImportPath);
-
-		// Replace them with files sourced from the live wp-content directory
-		const restoreFromPath = joinPaths(wpContentPath, relativePath);
-		if (await playground.fileExists(restoreFromPath)) {
-			await playground.mkdir(dirname(excludedImportPath));
-			await playground.mv(restoreFromPath, excludedImportPath);
-		}
-	}
-
 	// Carry over the database directory if the imported zip file doesn't
 	// already contain one.
 	const importedDatabasePath = joinPaths(