Removing NewTotalOrderPlainTableFactory

Summary:
Seems like NewTotalOrderPlainTableFactory is useless and is semantically incorrect.
Total order mode indicator is prefix_extractor == nullptr,
but NewTotalOrderPlainTableFactory doesn't set it to be nullptr. That's why some tests
in plain_table_db_tests is incorrect.

Test Plan: make all check

Reviewers: sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19587