Remove references to Value from LevelDB. It's not present in 1.13.

@michaelcahill When I tried to build the LevelDB API against 1.13, I saw the following:

```
../../../api/leveldb/leveldb_wt.cc:28:16: error: 'leveldb::Value' has not been declared
 using leveldb::Value;
                ^
../../../api/leveldb/leveldb_wt.cc:291:26: error: 'Value' has not been declared
        const Slice& key, Value* value);
                          ^
../../../api/leveldb/leveldb_wt.cc:501:25: error: 'Value' has not been declared
       const Slice& key, Value* value)
                         ^
../../../api/leveldb/leveldb_wt.cc: In member function 'virtual leveldb::Status DbImpl::Get(const leveldb::ReadOptions&, const leveldb::Slice&, int*)':
../../../api/leveldb/leveldb_wt.cc:515:9: error: request for member 'assign' in '* value', which is of non-class type 'int'
  value->assign((const char *)item.data, item.size);
         ^
```

I checked in the leveldb/db.h on my system (1.13) and ```leveldb::Value``` doesn't appear. Am I missing something?