gainmap.c: Omit unneeded avifImageAllocatePlanes

Omit the unneeded avifImageAllocatePlanes() call in
avifRGBImageComputeGainMap(). The avifImage is passed to
avifImageRGBToYUV(), which will call avifImageAllocatePlanes().

src/gainmap.c

diff --git a/src/gainmap.c b/src/gainmap.c
index a8d6288..e2e8b65 100644
--- a/src/gainmap.c
+++ b/src/gainmap.c
@@ -801,10 +801,6 @@
     gainMapImage->height = height;
 
     avifImageFreePlanes(gainMapImage, AVIF_PLANES_ALL); // Free planes in case they were already allocated.
-    res = avifImageAllocatePlanes(gainMapImage, AVIF_PLANES_YUV);
-    if (res != AVIF_RESULT_OK) {
-        goto cleanup;
-    }
 
     avifRGBImageSetDefaults(&gainMapRGB, gainMapImage);
     res = avifRGBImageAllocatePixels(&gainMapRGB);