The unexported function syscallMode was added in CL 47290 but was never
used anywhere. Remove it.

FWIW this should also fix the build failure on plan9, as the file in
question was lacking build tags.

Change-Id: Icfd6b2dbcd66ceb2065c945d8a5a6d55e634fe5e
Reviewed-on: https://go-review.googlesource.com/76510
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>