net: remove unused _C_GoString

net: remove unused _C_GoString

It's unused since CL 466335.

Change-Id: I2750f478bd2a1cde270a6273551e1434cb38b5ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/526076
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
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