unnecessary download of mxDataGenerator

[trixirt]

Download is done here https://github.com/ROCm/ROCmValidationSuite/blob/master/CMakeLists.txt#L391

before check of user setting inc dir here https://github.com/ROCm/ROCmValidationSuite/blob/master/CMakeLists.txt#L411

If the user passes in the variable like this -DMXDATAGENERATOR_INC_DIR=${PWD}/mxDataGenerator/lib/include

download is not needed. Also building the mxDataGenerator does not seem to be needed.