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 <[email protected]>
Reviewed-by: Filippo Valsorda <[email protected]>
Reviewed-by: Daniel McCarney <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Quim Muntal <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: qmuntal

src/runtime/runtime_boring.go

@@ -14,6 +14,3 @@ func boring_runtime_arg0() string {
 	}
 	return argslice[0]
 }
-
-//go:linkname fipstls_runtime_arg0 crypto/internal/boring/fipstls.runtime_arg0
-func fipstls_runtime_arg0() string { return boring_runtime_arg0() }