go/internal/gccgoimporter: remove workaround on Plan 9

go/internal/gccgoimporter: remove workaround on Plan 9

We fixed the implementation of the pread syscall in
the Plan 9 kernel, so calling pread doesn't update the
channel offset when reading a file.

Fixes #11194.

Change-Id: Ie4019e445542a73479728af861a50bb54caea3f6
Reviewed-on: https://go-review.googlesource.com/22245
Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>