Bug 33828: ExportCircHistory description is misleading

Bug 33828: ExportCircHistory description is misleading

This patch removes the mention of checkout history from the
ExportCircHistory system preference description, as this system
preference only allows to export current checkouts, not checkout
history.

This patch also modifies the description of the ExportRemoveFields
system preference to also remove mention of checkout history.

To test:
0. Apply patch
1. Go to Administration > System preferences > Circulation
2. Read the description for ExportCircHistory, make sure it makes sense,
   and that spelling and syntax are correct
3. Read the description for ExportRemoveFields, make sure it makes
   sense, and that spelling and syntax are correct

Signed-off-by: Sam Lau <samalau@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>