brew: remove explicit gpg-agent reference
brew: remove explicit gpg-agent reference

Apparently, you get this for free now with the gnupg formula. Remove it
because it was causing the "brew" role to error out.

https://stackoverflow.com/questions/52435650/gpg-agent-not-found-for-homebrew

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent