package/azure-iot-sdk-c: remove dummy value of CMAKE_SYSTEM_VERSION

Now CMAKE_SYSTEM_VERSION is properly set in toolchainfile.cmake, it is
no longer necessary to set a dummy value in azure-iot-sdk-c.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Nikita Sobolev <Nikita.Sobolev@synopsys.com>
Cc: André Hentschel <nerv@dawncrow.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>