[CUDA][HIP][NFC] Do not add `-fgpu-rdc` flag twice

Summary:
We currently add the `-fgpu-rdc` flag twice. Once unconditionally for
both the host and device phases of compilation, and a second time only
for the host. When we moved to an unconditional addition of this flag it
the old one was most likely not removed. This patch simply removes the
redundant flag and changes no functionality.