emacs: notmuch-mua-prompt-for-sender: don't force Ido on users

emacs: notmuch-mua-prompt-for-sender: don't force Ido on users

We shouldn't force `ido-completion-read' on users who do not otherwise
use Ido.  Unfortunately simply turning on `ido-mode' does not change
every `completing-read' into a `ido-completing-read', instead it only
changes file and buffer completion.

I do realize that existing Ido users will initially dislike this
change, but I would like to encourage them to see this as an
opportunity to learn about Fido.

Unlike `ido-mode', build-in `fido-mode' confirms to the standard
completion API, so turning it on causes every `completing-read' to
use the Fido completion mechanism and which is similar to the Ido
mechanism:

> An enhanced `icomplete-mode' that emulates `ido-mode'.  This global
> minor mode makes minibuffer completion behave more like `ido-mode'
> than regular `icomplete-mode'."