Also clean away impossible/unnecessary "Multi-Arch: same" stanzas.
Mostly upstreamed from https://salsa.debian.org/mariadb-team/mariadb-10.5/-/commit/f0ba31e1565feaca9c2a6f2534e55bfc29af7021

Omitted 'libzstd-dev (>= 1.3.3)' as the version requirement would need
stretch-backports to be available on buildbot.askmonty.org builders and
they are not yet.