runtime: remove redundant conversion
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 <2591e5f46f28d303f9dc027d475a5c60d8dea17a@kortschak.io>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@google.com>
Auto-Submit: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go