rm custom moe tune file. Add bash script for tuning reference (ROCm#305)

Since, rocm support has been added to the upstream tuning script, this PR removes the old custom tuning script "benchmark_mixtral_moe_rocm.py".
A bash script (moe_tune_script.sh) is also pushed as a reference on how to use the new tuning script for fp8 and fp16

Author: divakar-amd