Fix clang compile error - [-Werror,-Wunused-lambda-capture]

Summary:
Errors where:

db/version_set.cc:1535:20: error: lambda capture 'this' is not used [-Werror,-Wunused-lambda-capture]
                  [this](const Fsize& f1, const Fsize& f2) -> bool {
                   ^
db/version_set.cc:1541:20: error: lambda capture 'this' is not used [-Werror,-Wunused-lambda-capture]
                  [this](const Fsize& f1, const Fsize& f2) -> bool {
                   ^
db/db_test.cc:2983:27: error: lambda capture 'kNumPutsBeforeWaitForFlush' is not required to be captured for this use [-Werror,-Wunused-lambda-capture]
  auto gen_l0_kb = [this, kNumPutsBeforeWaitForFlush](int size) {
                          ^
Closes https://github.com/facebook/rocksdb/pull/1972

Differential Revision: D4685991

Pulled By: siying

fbshipit-source-id: 9125379

db/db_test.cc

@@ -2980,7 +2980,7 @@ TEST_F(DBTest, DynamicMemtableOptions) {
   options.level0_stop_writes_trigger = 1024;
   DestroyAndReopen(options);
 
-  auto gen_l0_kb = [this, kNumPutsBeforeWaitForFlush](int size) {
+  auto gen_l0_kb = [this](int size) {
     Random rnd(301);
     for (int i = 0; i < size; i++) {
       ASSERT_OK(Put(Key(i), RandomString(&rnd, 1024)));

db/version_set.cc

@@ -1537,13 +1537,13 @@ void VersionStorageInfo::UpdateFilesByCompactionPri(
         break;
       case kOldestLargestSeqFirst:
         std::sort(temp.begin(), temp.end(),
-                  [this](const Fsize& f1, const Fsize& f2) -> bool {
+                  [](const Fsize& f1, const Fsize& f2) -> bool {
                     return f1.file->largest_seqno < f2.file->largest_seqno;
                   });
         break;
       case kOldestSmallestSeqFirst:
         std::sort(temp.begin(), temp.end(),
-                  [this](const Fsize& f1, const Fsize& f2) -> bool {
+                  [](const Fsize& f1, const Fsize& f2) -> bool {
                     return f1.file->smallest_seqno < f2.file->smallest_seqno;
                   });
         break;