LP#1668720: remove browser console noise for OPAC

LP#1668720: remove browser console noise for OPAC

This patch fixes a case where JavaScript used in the OPAC
attempts to set the value of a #client_tz_id input on all
pages, even though only the login page has such an input.

To test:

[1] Load the OPAC homepage and open the JavaScript console/developer tools.
    Note that there's a "Cannot set property 'value' of null"
    error.
[2] Apply the patch and try again; this time there shouldn't
    be such an error.
[3] Open the login page, and verify, using developer tools,
    that the hidden client_tz_id input does have its value
    set to the client timezone.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>