benchmarks: move download_benchdata.bash into benchmarks/ dir

Change-Id: Ic47efde4a5c4dff204b6faca78c0d1ec99ec3aac
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/184944
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/download_benchdata.bash b/benchmarks/download_benchdata.bash
similarity index 100%
rename from download_benchdata.bash
rename to benchmarks/download_benchdata.bash