gnu: emacs-irony-mode: Remove propagated input

gnu: emacs-irony-mode: Remove propagated input

emacs-irony-mode finds its server binary because its a propagated input and is
in the path. We can substitute the variable `irony-server-install-prefix` to
not propagate the input and keep the binary isolated from the user.

* gnu/packages/emacs.scm (emacs-irony-mode):
[propagated-inputs] Remove propagated inputs
[inputs] Add emacs-irony-mode-server
[arguments] Add phase substituting the variable of the server path

Signed-off-by: Ludovic Courtès <ludo@gnu.org>