Remove very brittle __half complex hack/workaround!

[seberg]

After #145 the main CMakeLists.txt injects a header include for cpp/include/legate_dataframe/legion_hack which includes a legion_defines.h file with something like:

#include "legate/deps/legion_defines.h"

#ifdef LEGION_USE_CUDA
#undef LEGION_REDOP_HALF
#endif

This is nauseating of course. And also definitely very brittle. Any future errors around __half could be due to it!

This must be remove as soon as https://github.com/nv-legate/legate.internal/pull/1418 exists (or similar).