Remove the comments saying allow_concurrent_memtable_write and enable_write_thread_adaptive_yield are not stable

Summary: allow_concurrent_memtable_write and enable_write_thread_adaptive_yield went through extensive stress test and benchmarks. Should be considered stable.

Test Plan: N/A

Reviewers: ngbronson, yhchiang, IslamAbdelRahman, dhruba

Reviewed By: dhruba

Subscribers: MarkCallaghan, leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D59415

include/rocksdb/options.h

@@ -1266,8 +1266,6 @@ struct DBOptions {
   // It is strongly recommended to set enable_write_thread_adaptive_yield
   // if you are going to use this feature.
   //
-  // THIS FEATURE IS NOT STABLE YET.
-  //
   // Default: false
   bool allow_concurrent_memtable_write;
 
@@ -1276,8 +1274,6 @@ struct DBOptions {
   // This can substantially improve throughput for concurrent workloads,
   // regardless of whether allow_concurrent_memtable_write is enabled.
   //
-  // THIS FEATURE IS NOT STABLE YET.
-  //
   // Default: false
   bool enable_write_thread_adaptive_yield;