emacs: minimal change to load notmuch-tree by default

We want to load notmuch-tree when notmuch is loaded, so include it as
a require in notmuch.el. To avoid circular dependency we need to move
one keybinding from notmuch-tree.el to notmuch.el: it makes sense for
it to be defined there anyway.

Since tree is now loaded by default there is no need to print a
message when it is loaded.