chore: remove unused single step test flag (#1074)

## Changes

remove flag `single_step_test_show_before` from codebase

## Checks
- [x] deleted `single_step_test_show_before` from launch darkly
- [x] deleted `single_step_test_kill_switch ` from launch darkly

Author: pregnantboy

packages/frontend/src/config/flags.ts

@@ -5,8 +5,6 @@
  * Display flags
  */
 export const BANNER_TEXT_FLAG = 'banner_display'
-// we only want to show this notification to users created before this date (ms since epoch)
-export const SINGLE_STEP_TEST_SHOW_BEFORE_FLAG = 'single_step_test_show_before'
 
 /**
  * Feature flags