Now that MOZ_XBL has been removed (Bug 1593119), there is some code that needs to be cleaned up, as described by this Phabricator comment, the details of which I will reproduce below:

• Remove NODE_MAY_BE_IN_BINDING_MNGR. It does not appear to ever be set anymore.
• Is this still relevant? That is, can we instead just assert above that mParentAsSlot is not null and outdent? (Line)
• Without XBL, this always tests false. Arguably the ifdef here should have covered a larger block... We should simplify this code (Line)
• We should probably rename that member. (Line)
• Needs to be cleaned up. In particular, mIndexInInserted only gets set when mParentAsSlot is non-null now, so we can just assert that mIndexInInserted is 0. (Line)
• Again, can just assert mParentAsSlot is non-null. (Line)
• Again, can never be true now. (Line)
• This can just become: return FragmentOrElement::QueryInterface(aIID, aInstancePtr); (Line)
• This function should go away and its callsite get simplified. Also, DEBUG_CheckXBLLookup should go away. Not sure whether xpc::IsInContentXBLScope and the concept of "xbl scope" should go away or just get renamed. (Line)
• We should probably also have a followup to remove kNameSpaceID_XBL in general.
• This macro should just go away. (Line)
• This function should go away. (Line)

(In reply to Kirk Steuber (he/him) [:bytesized] from comment #0)

• We should probably also have a followup to remove kNameSpaceID_XBL in general.

Filed bug 1596478 for this one.