fix(mt): remove stale chunk-count local

Keep the MT chunk diagnostic after moving chunk iteration into Rust without
retaining a C local that is unused when debug logging is compiled out.

Test Plan:
- make -C lib -j2 lib
- git diff --cached --check

lib/compress/zstdmt_compress.c

@@ -774,9 +774,9 @@ static void ZSTDMT_compressionJob(void* jobDescription)
 
     /* compress the entire job by smaller chunks, for better granularity */
     {   size_t const chunkSize = 4*ZSTD_BLOCKSIZE_MAX;
-        int const nbChunks = (int)((job->src.size + (chunkSize-1)) / chunkSize);
         if (sizeof(size_t) > sizeof(int)) assert(job->src.size < ((size_t)INT_MAX) * chunkSize);   /* check overflow */
-        DEBUGLOG(5, "ZSTDMT_compressionJob: compress %u bytes in %i blocks", (U32)job->src.size, nbChunks);
+        DEBUGLOG(5, "ZSTDMT_compressionJob: compress %u bytes in %zu blocks",
+                    (U32)job->src.size, (job->src.size + (chunkSize-1)) / chunkSize);
         assert(job->cSize == 0);
         assert(chunkSize > 0);
         assert((chunkSize & (chunkSize - 1)) == 0);  /* chunkSize must be power of 2 for mask==(chunkSize-1) to work */