nextToUpdate3 does not need to be maintained outside of zstd_opt.c

It's re-synchronized with nextToUpdate at beginning of each block.
It only needs to be tracked from within zstd_opt block parser.

Made the logic clear, so that no code tried to maintain this variable.

An even better solution would be to make nextToUpdate3
an internal variable of ZSTD_compressBlock_opt_generic().
That would make it possible to remove it from ZSTD_matchState_t,
thus restricting its visibility to only where it's actually useful.

This would require deeper changes though,
since the matchState is the natural structure to transport parameters into and inside the parser.

lib/compress/zstd_lazy.c

@@ -653,7 +653,6 @@ size_t ZSTD_compressBlock_lazy_generic(
 
     /* init */
     ip += (dictAndPrefixLength == 0);
-    ms->nextToUpdate3 = ms->nextToUpdate;
     if (dictMode == ZSTD_noDict) {
         U32 const maxRep = (U32)(ip - prefixLowest);
         if (offset_2 > maxRep) savedOffset = offset_2, offset_2 = 0;
@@ -933,7 +932,6 @@ size_t ZSTD_compressBlock_lazy_extDict_generic(
     U32 offset_1 = rep[0], offset_2 = rep[1];
 
     /* init */
-    ms->nextToUpdate3 = ms->nextToUpdate;
     ip += (ip == prefixStart);
 
     /* Match Loop */