Remove TODO comment on cpi check in encoder_encode

The cpi != NULL check that the comment refers to has been removed. See
the corresponding libvpx CL:
https://chromium-review.googlesource.com/c/webm/libvpx/+/327699

Change-Id: I41054aaadce0993b1b098b3960e940ce24af3346
diff --git a/av1/av1_cx_iface.c b/av1/av1_cx_iface.c
index c48390ab..9ca6be4 100644
--- a/av1/av1_cx_iface.c
+++ b/av1/av1_cx_iface.c
@@ -2128,8 +2128,6 @@
 
   if (img != NULL) {
     res = validate_img(ctx, img);
-    // TODO(jzern) the checks related to cpi's validity should be treated as a
-    // failure condition, encoder setup is done fully in init() currently.
     if (res == AOM_CODEC_OK) {
       size_t data_sz = ALIGN_POWER_OF_TWO(ctx->cfg.g_w, 5) *
                        ALIGN_POWER_OF_TWO(ctx->cfg.g_h, 5) * get_image_bps(img);