remove unused return value of copy_back() in mariabackup

When does copy_back() return anything else than false? It seems that the variable ret is completely redundant. Highlighted by this:

/mariadb/10.4/extra/mariabackup/backup_copy.cc:1785:7: warning: 'ret' may be used uninitialized in this function [-Wmaybe-uninitialized]

 1785 |  bool ret;

Check if there is a code which expects return value from the function and verify it's correctness.