docs: remove dotenv from drizzle.config.ts examples

[f4rkh4d]

Closes #5725

drizzle-kit auto-loads .env since v0.20.0, so calling dotenv.config() inside drizzle.config.ts is no longer necessary.

This PR removes the dotenv import and config({ path: '...' }) call from the drizzle.config.ts snippets in 6 tutorials. The src/db/index.ts blocks still keep dotenv, since application code legitimately needs it at runtime.

Files touched:

• drizzle-with-neon
• drizzle-with-supabase
• drizzle-with-turso
• drizzle-with-vercel
• drizzle-with-xata
• drizzle-nextjs-neon

18 deletions total, no other changes.