emacs: do not modify subject in search or show

A previous patch [0] replaced blank subject lines with '[No Subject]'
in search and show mode.  Apparently this was needed to circumvent
some bug in the printing code, but there was no need for it search or
show, and it is definitely not desirable, so we undo it here (a revert
is no longer feasible).  We should not be modifying strings in the
original message without good reason, or without a clear indication
that we are doing so, neither of which apply in this case.  For
further discussion see [0].

[0] id:"1327918561-16245-3-git-send-email-dme@dme.org"