Nodes: revert the inline (pass-through) socket feature

Inlined sockets in the same vertical space are no longer supported.
This removes `input_output` socket declarations, the inlining feature in
node drawing, and the `Both` option for node group interface sockets.

Versioning code splits existing node group sockets into individual
sockets again. Unfortunately some links may get lost in versioning files
using the feature, because of an unnoticed bug: Socket identifiers have
to be unique in the node group items list but inlined input/output
sockets have the same identifier. This still works for most situations
because uniqueness is only required within input/output lists. Creating
proper unique identifiers will discard any link from the previous output
socket. This cannot easily be fixed without `after_linking` versioning
code, which should be avoided.

Pull Request: https://projects.blender.org/blender/blender/pulls/112560

tests/python/bl_node_group_interface.py

@@ -132,18 +132,13 @@ class NodeGroupInterfaceTests:
         out1 = tree.interface.new_socket("Output 1", socket_type=socket_type, in_out='OUTPUT')
         self.assertIsNotNone(out1, f"Could not create socket of type {socket_type}")
 
-        inout0 = tree.interface.new_socket("Input/Output 0", socket_type=socket_type, in_out='BOTH')
-        self.assertIsNotNone(inout0, f"Could not create socket of type {socket_type}")
-
         self.assertSequenceEqual([(s.name, s.bl_idname) for s in group_node.inputs], [
             ("Input 0", socket_type),
             ("Input 1", socket_type),
-            ("Input/Output 0", socket_type),
         ])
         self.assertSequenceEqual([(s.name, s.bl_idname) for s in group_node.outputs], [
             ("Output 0", socket_type),
             ("Output 1", socket_type),
-            ("Input/Output 0", socket_type),
         ])
 
     def do_test_user_count(self, value, expected_users):