chore(#620): drop unused stellar-sdk dependency

Every import in backend/src uses @stellar/stellar-sdk; the bare stellar-sdk
package was never referenced and added unnecessary bloat.

Author: Fury03

backend/package.json

@@ -32,7 +32,6 @@
     "express-rate-limit": "^8.5.2",
     "ioredis": "^5.11.1",
     "pg": "^8.21.0",
-    "stellar-sdk": "^13.3.0",
     "swagger-jsdoc": "^6.3.0",
     "swagger-ui-express": "^5.0.1",
     "winston": "^3.11.0",