Remove explicit type conversion panic.

This commit removes an explicit panic on a failed type conversion,
when the language provides this behavior by default.
Remove naked returns from leveldb/block/filter.go.
Remove naked returns from leveldb/block/reader.go.
Remove naked returns from leveldb/cache/empty_cache.go.
Remove naked returns from leveldb/cache/lru_cache.go.
Remove naked returns from leveldb/db.go.
Remove naked returns from leveldb/db_snapshot.go.
Remove naked returns from leveldb/db_state.go.
Remove naked returns from leveldb/db_test.go.
Remove naked returns from leveldb/db_util.go.
Remove naked returns from leveldb/db_write.go.
Remove naked returns from leveldb/journal/reader.go.
Remove naked returns from leveldb/journal/writer.go.
Remove naked returns from leveldb/key.go.
Remove naked returns from leveldb/memdb/memdb.go.
Remove naked returns from leveldb/session.go.
Remove naked returns from leveldb/session_record.go.
Remove naked returns from leveldb/session_util.go.
Remove naked returns from leveldb/storage/file_storage.go.
Remove naked returns from leveldb/storage/file_storage_unix.go.