syscall: remove unused ptracePtr on openbsd

syscall: remove unused ptracePtr on openbsd

ptracePtr was introduced in CL 470299 for openbsd but it's not used on
this platform.

For #58387

Change-Id: I27901c3f4497e18dc407a9248b66e2691c9a6abb
Reviewed-on: https://go-review.googlesource.com/c/go/+/527535
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