emacs: Don't override mm-show-part in notmuch-show-view-part

emacs: Don't override mm-show-part in notmuch-show-view-part

Previously, notmuch-show-view-part overrode the function binding of
mm-show-part to redirect it to notmuch-show-save-part to get notmuch's
default file name handling in case mm-display-part decided to fall
back to saving the part.  In addition to being messy, this depended on
the now-deprecated dynamic binding behavior of flet.

This patch removes the mm-show-part override in favor of passing the
file name in to mm-show-part the way it expects, so we get its default
file name handling.  It's not clear why we didn't do this before;
mm-show-part has supported default file names since at least Emacs
23.1.