Drop RootNode extraction option
We never set this option and because of this there is a lot of unused code (e.g. all the RootNodeSwitch stuff).
Digging in the history, the last time that this option was set was in commit 10bc72f453a253.
This means that this option is fully superseded by fileNodeRootPair and we can safely discard all this unused code.
Closes T1942
Differential Revision: D1704