This commit removes the OpenSSL dependency unconditionally from
external/libxmlsec/ExternalProject_xmlsec.mk.
xmlsec wasn't built for Android previously but it seems like
Android was the only user of xmlsec OpenSSL backend.
Now we have NSS built for Android as well, so it's safe to remove
the OpenSSL dependency.

Change-Id: I8652487be146c3ac643c4f6dc7a6a2845093c93f
Reviewed-on: https://gerrit.libreoffice.org/41288Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>