Remove highlight for root and GLOBAL keywords

[BaliBalo]

These are currently highlighted differently than other variable names because of this line.

They were never documented in the first place, and deprecated when moving to Node v6 for which support has been dropped quite a long time ago now.

• archive of breaking changes list from v5 to v6
• reference in the current changelog (less explicit but listed under commit 4e46931406, named "deprecate undocumented variables")
• initial pull request for the removal

I believe this makes it quite superfluous to highlight them, and just becomes slightly confusing when trying to create a variable with that name (can make people think root is some kind of reserved keyword for example).