developer_attribute_composite: the Attribute menu is an expression

Houdini evaluates a one-line Python menu script as an expression, so
the "return ..." on the multiparm's in_attr_# field raised a
SyntaxError popup every time the parameter pane built the dropdown.
Use the bare expression the other attribute menus use.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

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