Bug 8701: Remove links to OpacHiddenItems.txt in favor of wiki page

Bug 8701: Remove links to OpacHiddenItems.txt in favor of wiki page

Removes the OpacHiddenItems.txt file from Koha as this is not
accessible for most people and replaces it with links to the wiki
page like it's already done from the system preference editor.

For reporting and the initial patch
Thanks-to: Jane Sandberg <sandbej@linnbenton.edu>
To test:
- Changes are invisible from the GUI
- Apply patch and run database update
- SELECT * FROM systempreferences WHERE variable = "OpacHiddenItems";
- Verify the description has the wiki link
- Verify that docs/opac/OpacHiddenItems.txt has been removed

Signed-off-by: Hayley Mapley <hayleymapley@catalyst.net.nz>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>