test: Remove refcnttest from the unittest SConscript.

test: Remove refcnttest from the unittest SConscript.

The test itself was removed, but it was left in the SConscript. If you
tell scons to build everything in that directory, it will try to build
that test and fail.

Change-Id: I1e3923b0de12e891f53dab6f4e6e3e2b6975dc45
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33896
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>