This is mostly just the offapi/ part, more C++ code can be removed in
xmlsecurity/ after this. XMLEncryption wasn't used by internal code
since it was added in commit 9f34fde7
(libxmlsec, 2004-07-12).

It seems to be unlikely that this non-published service was used by
3rd-party code, since it was specific to other xmlsecurity/ code, not
generic functionality that could be useful in other contexts.

Change-Id: Ia0168f67d1c8ff7a019760ac436511f219eee94e
Reviewed-on: https://gerrit.libreoffice.org/40162Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>