OPENDJ-2730 Fix upgrade on case insensitive file systems

Chris spotted that since ed3930d, the upgrade removes opendj.jar and
opendj_*.jar from case insensitive file systems (case preserving fs are not impacted).
This is caused by the "cleanup lib directory"  upgrade task which wants to delete old OpenDJ
camel case jars which have been overridden by "valid" lower case jars on
case insensitive/preserving file systems.
On case preverving file systems, the delete fails.
On case insensitive file systems, it works.
This commit fixes the issue by checking whether the file system is case
sensitive before removing old OpenDJ[_*].jar jars.
The fix does not concern OpenDJ[-*].jar jars because the have to be
removed anyway (see OPENDJ-2692).