Remove unnecessary header include

Summary:
Remove "util/testharness.h" from list of includes for "db/db_filesnapshot.cc", as it wasn't being used and thus caused an extraneous dependency on gtest.
Closes https://github.com/facebook/rocksdb/pull/1634

Differential Revision: D4302146

Pulled By: yiwu-arbug

fbshipit-source-id: e900c0b

db/db_filesnapshot.cc

@@ -27,7 +27,6 @@
 #include "util/file_util.h"
 #include "util/mutexlock.h"
 #include "util/sync_point.h"
-#include "util/testharness.h"
 
 namespace rocksdb {