Drop Snow codec

Snow is a toy codec with no real-world use and horrible code.

libavcodec/allcodecs.c

@@ -212,7 +212,6 @@ void avcodec_register_all(void)
     REGISTER_ENCDEC (SGI,               sgi);
     REGISTER_DECODER(SMACKER,           smacker);
     REGISTER_DECODER(SMC,               smc);
-    REGISTER_ENCDEC (SNOW,              snow);
     REGISTER_DECODER(SP5X,              sp5x);
     REGISTER_ENCDEC (SUNRAST,           sunrast);
     REGISTER_ENCDEC (SVQ1,              svq1);