build: avoid unused-variable warning
build: avoid unused-variable warning

Summary:
[noticed a new warning when building with the very latest gcc]
* db/memtablerep_bench.cc (FLAGS_env): Remove declaration
of unused varaible, to avoid this warning/error:

db/memtablerep_bench.cc:135:22: error: ‘FLAGS_env’ defined but not\
  used [-Werror=unused-variable]
 static rocksdb::Env* FLAGS_env = rocksdb::Env::Default();
                      ^

Test Plan: compile

Reviewers: ljin, rven, igor.sugak, yhchiang, sdong, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D37983

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