doc: fix rollback in 5.2-to-2023.1 upgrade guide

This commit fixes the Restore System Tables section
in the 5.2-to-2023.1 upgrade guide by adding a command
to clean upgraded SStables during rollback.

This is a bug (an incomplete command) and must be
backported to branch-5.3 and branch-5.2.

Refs: https://github.com/scylladb/scylla-enterprise/issues/3046

Closes #14373

(cherry picked from commit f4ae2c09)