delete array mdadm

your array is mounted on /dev/md0 with two devices /dev/sda and /dev/sdb.

Unmount you sweety:

umount /dev/md0

Stop array:

mdadm --stop /dev/md0

Reset your drives:

mdadm --zero-superblock /dev/sda
mdadm --zero-superblock /dev/sdb

Source:
https://ubuntuforums.org/showthread.php?t=884556