runtime: remove redundant conversion
This appears to have been left over from a C cast during the rewrite of
malloc into Go in https://golang.org/cl/
108840046.
Change-Id: I88f212089c2bcf79d5881b3e8bf3f94f343331d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/443235
Run-TryBot: Dan Kortschak <dan@kortschak.io>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>