Bug 18781: Translatability: Get rid of exposed tt directives in openlibrary-readapi.inc

Bug 18781: Translatability: Get rid of exposed tt directives in openlibrary-readapi.inc

The file koha-tmpl/opac-tmpl/bootstrap/en/includes/openlibrary-readapi.inc
exposes template directives to translation. The only string that should
appear in .po from this file is "Open Library: "

To test:
- Apply patch
- Verify that code changes make sense
- Bonus test: create a new language 'aa-AA', verify in aa-AA-opac-bootstrap.po
  that there is only the following string for openlibrary-readapi.inc:
msgid "Open Library: "
msgstr ""

NOTE: Followed a test plan similar to bug 18776 comment 3

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 23cc8b39682fea7b0a9150933c65c34ef22d23dd)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>