fix(Dashboard): Exclude edit param in async screenshot

[geido]

SUMMARY

This PR https://github.com/apache/superset/pull/30675 introduced a way to pass urlParams to the async dashboard screenshot generation endpoint. However, edit was an acceptable value which caused the system to take a screenshot in edit mode of a newly created dashboard. This PR fixes that.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N.A.

TESTING INSTRUCTIONS

1. Create a new dashboard
2. Download the dashboard screenshot as PDF
3. The screenshot should not be showing the dashboard in edit mode

ADDITIONAL INFORMATION

• [ ] Has associated issue:
• [x] Required feature flags: ENABLE_DASHBOARD_SCREENSHOT_ENDPOINTS ENABLE_DASHBOARD_DOWNLOAD_WEBDRIVER_SCREENSHOT
• [ ] 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