280: Remove unused properties from XPlaneLayer, elsewhere

[tngreene]

Apparently

    # Deprecated: This will be removed in v3.4
    cockpit: bpy.props.BoolProperty(
        name = "Cockpit",
        description = "If checked the exported object will be interpreted as a cockpit",
        default = False
    )

this was supposed to be removed a long long time ago. Its not hurting anything, but, I don't want any extraneous things hanging around in the data model, lest someone get an idea.

• [ ] Updater unit test
• [ ] Updater code to delete it

[tngreene]

See also litFactor

[tngreene]

Now that we're not deleting old idprops this bug is really just deleting lines of text