Revert "Add an __UNDEFINED_ value to all ProtoEnums"

This reverts commit 59288e6b539057a8e1317a3b15805a53cf4db067 and 5a97a3788bddae5fddf7619b65affe83d80a9f60.
Merge pull request #158 from square/jw/revert-undefined

Revert "Add an __UNDEFINED_ value to all ProtoEnums"