Remove `custom` scan action from Scan execution policy schema

Why are we doing this work

In #472193 (closed) we cleaned up the code for the ended experiment, but we kept the schema so that we don't invalidate all policies. In %18.0, we should remove the custom scan action from the schema.

Relevant links

Non-functional requirements

  • Documentation:
  • Feature flag:
  • Performance:
  • Testing:

Implementation plan

!161809 (closed)