fs_mgr: removing is_device_secure()

Previously there is no vboot 1.0 metadata for ENG builds. It relies on
is_device_secure() to query "ro.secure" and skip setting up dm-verity
if the value is 0 (meaning ENG build).

This change will be submitted together with other changes to add vboot
1.0 metadata for ENG builds with a "disable magic". The resulting
metadata will be the same as triggering an "adb disable-verity" on an
USERDEBUG image.

Bug: 63056044
Test: boot sailfish eng/userdebug builds

Change-Id: I35eef771e1b30bfc6d01b8ed76b40c942fe7b783