Remove default export

[vweevers]

Originally added for TypeScript folks. See Level/level#170 (comment).

• encoding-down: Level/encoding-down@1111866
• memdown: Level/memdown@5fd8368
• leveldown: Level/leveldown@686d685
• rocksdb: Level/rocksdb@43c1544
• levelup: Level/levelup#716
• level-js (n/a)
• level (n/a)
• packager (n/a)
• subleveldown (n/a)
• deferred-leveldown (n/a)
• abstract-leveldown (n/a)

[vweevers]

Done