[NFC] Fixing redundant discard assign void type warn on stdlib (#30397)

Author: LucianoPAlmeida

stdlib/public/Darwin/Accelerate/vImage_Buffer.swift

@@ -217,7 +217,7 @@ extension vImage_Buffer {
 
         var error = kvImageNoError
 
-        _ = withUnsafePointer(to: self) {
+        withUnsafePointer(to: self) {
             error =  vImageCopyBuffer($0,
                                       &destinationBuffer,
                                       pixelSize,

stdlib/public/Darwin/Accelerate/vImage_Converter.swift

@@ -224,7 +224,7 @@ extension vImageConverter {
 
         var error = kvImageNoError
 
-        _ = withUnsafePointer(to: source) { src in
+        withUnsafePointer(to: source) { src in
             error = vImageConvert_AnyToAny(self,
                                            src,
                                            &destination,