Remove PICK_MODE_CONTEXT::skip_txfm.

It is currently unused.

Change-Id: Iba52a4926e83411f3c454ed83a615652be709157

av1/encoder/context_tree.h

@@ -47,7 +47,6 @@ typedef struct {
   // For current partition, only if all Y, U, and V transform blocks'
   // coefficients are quantized to 0, skippable is set to 0.
   int skippable;
-  uint8_t skip_txfm[MAX_MB_PLANE << 2];
   int best_mode_index;
   int hybrid_pred_diff;
   int comp_pred_diff;