fix GuestDevice#child_device references

[kbrock]

The child_devices added unneeded scope (and therefore an extra GuestDevices.pluck(:id))

Before

has_many :child_devices, -> { where(:parent_device_id => ids) }, :foreign_key => "parent_device_id"
# rewritten to make the issue more obvious: 
has_many :child_devices, -> { where(:parent_device_id => GuestDevice.ids) }, :foreign_key => "parent_device_id"

SELECT "guest_devices".*
FROM "guest_devices"
WHERE "guest_devices"."parent_device_id" = 26000000000035
  AND "guest_devices"."parent_device_id" IN (26000000000032, 26000000000033, 26000000000034, 26000000000035, 26000000000036, 26000000000037, 26000000000038)

After

has_many :child_devices, :foreign_key => "parent_device_id"

SELECT "guest_devices".*
FROM "guest_devices"
WHERE "guest_devices"."parent_device_id" = 26000000000035

This was introduce in #15371
The way the PR was batted around, it looked like this was not an intentional side effect.

[kbrock]

update:

• fixed cap: useless assignment in spec

update:

• added parent_device to make rubocop happy

[kbrock]

update:

• rebased (updated puma) to fix build

[miq-bot]

Checked commits kbrock/manageiq@7299d1a~...732039f with ruby 2.6.10, rubocop 1.28.2, haml-lint 0.35.0, and yamllint
2 files checked, 0 offenses detected
Everything looks fine. 👍