emacs: Fix to unconditionally display subject changes in collapsed thread view

emacs: Fix to unconditionally display subject changes in collapsed thread view

The feature to show subject changes in the collapsed thread view was
originally added (8ab433607) with an option
(notmuch-show-always-show-subject) to display the subject
for all messages, even when there was no change.

The subsequent commit (4f04d273) changed the sense of the test (or to
and) and the name of the controlling variable
(notmuch-show-elide-same-subject).

But this commit is broken in a few ways:

  1. The original definition of notmuch-show-always-show-subject was
     left around

     But the variable isn't actually used in the code at all, so it
     just adds clutter and confusion to the customization interface.

  2. The name and description of the controlling variable doesn't
     match the implementation

     The name suggests that setting the variable to t will cause
     repeated subjects to be elided, (suggesting that when it is nil
     all subjects will be shown).

     However, when the variable is nil, no subjects are shown. So a
     correct name for the variable in this sense would be
     notmuch-show-subject-changes.

Showing subject changes is a useful feature, and should be on by
default. (We don't want to bury generally useful features behind
customizations that users have to find).

Rather than fixing the name of the variable and changing its default
value, here we remove the condition entirely, such that the feature is
enabled unconditionally.

So both the currently-used variable and the stale definition of the
formerly-used are removed.

Also, the one relevant test-suite result is updated, (showing the
intial subject of a collapsed thread, and no subject display for later
messages that do not change the subject).