Bug 34584: Remove Twitter share button from the OPAC

Bug 34584: Remove Twitter share button from the OPAC

This patch removes the option to share content in the OPAC via Twitter
now that Twitter doesn't exist anymore.

To test, apply the patch and go to Administration -> System preferences.

- Search for "SocialNetworks."
- The menu of options should contain only email, Facebook, and LinkedIn.
- Select all options and save.
- In the OPAC, locate a bibliographic record and view its details.
- In the right-hand sidebar, you should only see options to share via
  email, Facebook, and LinkedIn.

Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>