tests:os: Avoid setting apiEndpoint for unmanaged tests

[klutchell]

When this config.json field has a value, the supervisor will treat the device as managed and will crash and start to boot loop.

On faster device-types we don't seem to notice and the tests pass by chance, but on Pi Zero and similar we consistently see ECONNRESET when hitting local SV endpoints.

Change-type: patch
See: #3510
Added-in: #2369 and #3022

Warning

This REVERTS the changes made in #3022 and we need to retest the flashing of Jetson TX2 devices.

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[acostach]

lgtm