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.

[jkottiku]

@trixirt mxDataGenerator is header-only library so we don't build it separately rather it is done during RVS compilation. Since mxDataGenerator currently is a separate github project and now part of ROCm code base we are downloading it.