Fix compile error when LZ4 is up to r123 (#6412)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/6412

Differential Revision: D19914063

Pulled By: ajkr

fbshipit-source-id: 4e401e665d4b449d24c4cdec35a4585eeda95996

util/compression.h

@@ -1050,7 +1050,6 @@ inline bool LZ4_Compress(const CompressionInfo& info,
 #else   // up to r123
   outlen = LZ4_compress_limitedOutput(input, &(*output)[output_header_len],
                                       static_cast<int>(length), compress_bound);
-  (void)ctx;
 #endif  // LZ4_VERSION_NUMBER >= 10400
 
   if (outlen == 0) {
@@ -1115,7 +1114,6 @@ inline CacheAllocationPtr LZ4_Uncompress(const UncompressionInfo& info,
   *decompress_size = LZ4_decompress_safe(input_data, output.get(),
                                          static_cast<int>(input_length),
                                          static_cast<int>(output_len));
-  (void)ctx;
 #endif  // LZ4_VERSION_NUMBER >= 10400
 
   if (*decompress_size < 0) {