Remove incorrect comment

Summary:
We actually create individual compaction filter from compaction filter factory per sub-compaction in `CompactionJob::ProcessKeyValueCompaction`: https://github.com/facebook/rocksdb/blob/master/db/compaction_job.cc#L742
The comment seems incorrect.
Closes #3288

Differential Revision: D6598455

Pulled By: yiwu-arbug

fbshipit-source-id: a6bc059a9103b87a73ae6ec4bb01ca33f5d48cf5

include/rocksdb/compaction_filter.h

@@ -94,10 +94,6 @@ class CompactionFilter {
   // be used by a single thread that is doing the compaction run, and this
   // call does not need to be thread-safe.  However, multiple filters may be
   // in existence and operating concurrently.
-  //
-  // The last paragraph is not true if you set max_subcompactions to more than
-  // 1. In that case, subcompaction from multiple threads may call a single
-  // CompactionFilter concurrently.
   virtual bool Filter(int level, const Slice& key, const Slice& existing_value,
                       std::string* new_value, bool* value_changed) const {
     return false;