BUG: Drop the dead non-interactive test override

The script was never wired into anything: the only reference to it in
the kit was its own header comment.

It also cannot work the way that comment describes. It documents
injection at Slicer startup, but qSlicerCoreCommandOptions declares
--testing to imply --disable-settings and --ignore-slicerrc, so SLICERRC
is not read for exactly the runs it targeted; PYTHONPATH is likewise
reset by the launcher. Both were confirmed empirically.

Extension self-tests are off by default (SLICER_EXT_RUN_TESTS), so
nothing needs it. Modules that pop a modal dialog under --testing should
guard it with slicer.app.testingEnabled().

Author: hjmjohnson