d/rules: drop Ubuntu-specific configure option

Ubuntu used to use the bundled libssh2 copy because libssh2 from the archive is in Universe, and nmap used to be in main.

Since https://code.launchpad.net/~paelzer/ubuntu-seeds/+git/ubuntu/+merge/376744 nmap is in universe, so there is no issue in linking nmap with libssh2 now.

Furthermore, since libssh2 was removed from the tarball, that configure option is causing nmap to FTBFS in Ubuntu.