runtime: remove unused fipstls_runtime_arg0 function

runtime: remove unused fipstls_runtime_arg0 function

fipstls_runtime_arg0 is unused since CL 629675.

Change-Id: I981e55e95f4d1cbd33f4c438c1947ec39385376b
Reviewed-on: https://go-review.googlesource.com/c/go/+/631215
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>