emacs: Rip out all of the notmuch-folder code.

emacs: Rip out all of the notmuch-folder code.

We are asserting that the new notmuch-hello implementation, (available
by just calling `notmuch') is just as easy to use as the old
notmuch-folder. So let's remove what's now a largely redundant
implementation.

To make this transition easier, we are still supporting the
notmuch-folders variable name, and we still provide `notmuch-folder'
as an alias which can be invoked to get the new notmuch-hello
functionality.