Remove subform in subform limitation for custom fields

[HLeithner]

Summary of Changes

Remove the limitation to select a subform as a subform field in custom fields.

There is no reason we have this limit, subforms can already be used as field of another subform.

Testing Instructions

• Create a number of custom fields
• Create a subform !1
• Add the fields to the subform
• Create a subform !2
• Add subform !1 to subform !2
• check if everything works

Actual result BEFORE applying this Pull Request

It's not possible to select subform !1 in subform !2

Expected result AFTER applying this Pull Request

You can select subform !1 in subform !2 and you also can edit it

[wilsonge]

I think we need to consider a nesting limit to stop the UI from falling apart.

[brianteeman]

a limit of zero - aka the current situation seems perfect to me

[HLeithner]

I think we need to consider a nesting limit to stop the UI from falling apart.

didn't looked at it yet but someone already came up with the question and I hope this protection is already in the field^^

[wilsonge]

I think that protection is the code your deleting ;)

[crystalenka]

I would be very happy to have this in core, personally.

[HLeithner]

This pull request has been automatically rebased to 4.3-dev.

[HLeithner]

This pull request has been automatically rebased to 4.4-dev.

[Quy]

Alternate #43204 that appears to be more complete.

[Quy]

Closing in favor of #43204. Thanks.