chore: Remove redundant validation functions

[sebastianliebscher]

SUMMARY

This PR removes redundant validate_json functions and replaces usage with already existing superset.utils.schema.

TESTING INSTRUCTIONS

• local unit / integration tests + GitHub CI

ADDITIONAL INFORMATION

• [ ] Has associated issue:
• [ ] Required feature flags:
• [ ] Changes UI
• [ ] Includes DB Migration (follow approval process in SIP-59)
  • [ ] Migration is atomic, supports rollback & is backwards-compatible
  • [ ] Confirm DB migration upgrade and downgrade tested
  • [ ] Runtime estimates and downtime expectations provided
• [ ] Introduces new feature or API
• [ ] Removes existing feature or API