Now backup and restore operations free logger resources calling
to the destroy() method. This commit ensures:
1) That gc_collect_cycles() is not used anymore in backup-related tests.
2) That all backup and restore controllers in test do always call
to the detroy() method.
3) Some unset() calls, needed to make gc_collect_cycles() are not used
anymore.
Without the patch a course would not be created if it contained
an enrolment method column with an empty value. e.g.:
shortname,fullname,category,enrolment_1,enrolment_1_disable
courserestored,Course restored,1,,
courserestored2,Course restored 2,manual,1
