Remove select_tx_size from struct macroblock

It is no longer used anywhere.

Change-Id: I5d40664373d66821c5382f6155302b8719ce58c0

av1/encoder/block.h

@@ -80,7 +80,6 @@ struct macroblock {
   MACROBLOCKD e_mbd;
   MB_MODE_INFO_EXT *mbmi_ext;
   int skip_block;
-  int select_tx_size;
   int qindex;
 
   // The equivalent error at the current rdmult of one whole bit (not one