row_vers_vc_matches_cluster(): Remove the parameter in_purge,
which was always passed as in_purge=true.

This parameter became constant in
mysql/mysql-server@1dec14d346ac55fe72989dccb071f84b3b0d3bd6
and it always was constant in MariaDB starting from the
introduction of the function in
commit 2e814d47 (MariaDB 10.2.2).