syscall: rm obsoleted comment

syscall: rm obsoleted comment

The comment being removed is no longer true as of CL 494916.

Change-Id: I4975866c58efb5c9c41db3640564ac7233f5840f
Reviewed-on: https://go-review.googlesource.com/c/go/+/542415
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Reviewed-by: Than McIntosh <thanm@google.com>
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Reviewed-by: Bryan Mills <bcmills@google.com>
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