Drop restrict_user_defined_variables

Why are we doing this work

As a result of internal deprecation of restrict_user_defined_variable - !189388 (comment 2554570941), we have to drop it in the DB, after it was ignored in %18.1 (!189388 (merged))

Relevant links

Related: #514242 (closed)

Non-functional requirements

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

Implementation plan

Verification steps