avcodec/libx264: remove NV21, its not supported

Regression introduced in 8d75aa8d79519c21f91a7dd96f330ad30d6625ed

Found-by: jamrial
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
diff --git a/libavcodec/libx264.c b/libavcodec/libx264.c
index 7f46abd..72427d3 100644
--- a/libavcodec/libx264.c
+++ b/libavcodec/libx264.c
@@ -747,9 +747,6 @@
     AV_PIX_FMT_YUVJ444P,
     AV_PIX_FMT_NV12,
     AV_PIX_FMT_NV16,
-#ifdef X264_CSP_NV21
-    AV_PIX_FMT_NV21,
-#endif
     AV_PIX_FMT_YUV420P10,
     AV_PIX_FMT_YUV422P10,
     AV_PIX_FMT_YUV444P10,