Delete samples directory

[NimJay]

• The Developer Enablement Engineering (DEE) team is currently migrating code samples from GitHub repos such as googleapis/nodejs-secret-manager to the monorepo GoogleCloudPlatform/nodejs-docs-samples.
• See Google-internal Buganizer at b/257074860.
• The code samples from the samples/ folder have already been migrated to GoogleCloudPlatform/nodejs-docs-samples/tree/main/secret-manager/.
• I have made the appropriate documentation updates inside Google-internal cl/489558049.