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 <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: kortschak

src/runtime/malloc.go

@@ -1043,7 +1043,7 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
 			}
 			x = unsafe.Pointer(v)
 			if needzero && span.needzero != 0 {
-				memclrNoHeapPointers(unsafe.Pointer(v), size)
+				memclrNoHeapPointers(x, size)
 			}
 		}
 	} else {