Documentation Request Summary:

upgrade_check.js, which this commit deleted, contained the following functions which are documented for v2.6 use:

• upgradeCheckAllDBs (https://docs.mongodb.com/manual/reference/method/db.upgradeCheckAllDBs/)
• upgradeCheck (https://docs.mongodb.com/manual/reference/method/db.upgradeCheck/)

We no longer use these functions internally anywhere, and they have no external utility since a v3.6 shell will be unable to communicate with v2.6 servers where the functions are relevant.

Engineering Ticket Description:

shell/upgrade_check.js is 2.6->3.0 upgrade testing code and no longer used.

mozjs/object.h/cpp defines a function invalidForStorage, which is used soley by shell/upgrade_check.js. This code can be deleted

This task is part of resolving SERVER-29334 (cleanup). mozjs/object.h/cpp uses BSONObj::storageValid here, which needs to be modified to resolve SERVER-29334. This cleans up an unnecessary use case.