Remove the code that support namespace strategies other than "per user"

[skabashnyuk]

Is your task related to a problem? Please describe.

In 7.29 #19365 we deprecated all namespace strategies other than "per user".
In 7.31 we want to left only the "per-user" strategy in the code.

Describe the solution you'd like

Remove/Cleanup the code that supports namespace strategies other than "per user"

Describe alternatives you've considered

n/a

Additional context

#19365