Fix SIGFAULT when running sst_dump on v2.6 db

Summary: Fix the sigfault when running sst_dump on v2.6 db.

Test Plan:
    git checkout bba6595b1f
    make clean
    make db_bench
    ./db_bench --db=/tmp/some/db --benchmarks=fillseq
    arc patch D18039
    make clean
    make sst_dump
    ./sst_dump --file=/tmp/some/db --command=check

Reviewers: igor, haobo, sdong

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18039

table/block_based_table_reader.h

@@ -199,8 +199,4 @@ class BlockBasedTable : public TableReader {
   void operator=(const TableReader&) = delete;
 };
 
-// Backward compatible properties block name. Limited in block based
-// table.
-extern const std::string kPropertiesBlockOldName;
-
 }  // namespace rocksdb