Revert "mdadm: update from 3.3 to 3.3.2"

This reverts commit 1faf6106c3fc5890297ad6c8e26b572d91dede5a. It
causes the size of the initrd to explode from ~6 MB to 19 MB:

  http://hydra.nixos.org/build/15728196

The reason is that mdadm now contains a self-reference:

  $ strings ./result/sbin/mdadm  | grep /nix/store
  ...
  ACTION=="add", SUBSYSTEM=="block", ENV{DEVTYPE}=="%s", ENV{ID_PATH}=="%s", RUN+="/nix/store/y1ap6va0av6jhxf7ddnr8vdwxd7b44gd-mdadm-3.3.2/sbin/mdadm --incremental $env{DEVNAME}"
  ACTION=="add", SUBSYSTEM=="block", ENV{ID_PATH}=="%s", RUN+="/nix/store/y1ap6va0av6jhxf7ddnr8vdwxd7b44gd-mdadm-3.3.2/sbin/mdadm --incremental $env{DEVNAME}"
/nix/store/y1ap6va0av6jhxf7ddnr8vdwxd7b44gd-mdadm-3.3.2/sbin/mdmon

(Previously, it referred to /sbin/mdadm.) This causes mdadm, glibc and
linux-headers to be added to the initrd.