Bug 28927: Fix id opacmainuserblock used twice in OPAC

Bug 28927: Fix id opacmainuserblock used twice in OPAC

Since preference OpacMainUserBlock is now an HTML customization,
there is actually an HTML tags with id="opacmainuserblock" and one with id="OpacMainUserBlock".
Same for OPACNavRight in opac-registration-confirmation.tt

This patch removed id from page to keep the one comming from HTML customization include.

Test plan :
1) Create an HTML customization for OpacMainUserBlock with some text
2) Go to OPAC main page
3) Check HTML code of the page
4) Check you see id="OpacMainUserBlock" once and not id="opacmainuserblock"
5) Same for id="opacnavright" in opac-registration-confirmation.tt

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>