emacs: notmuch-search-stash-thread-id: use notmuch-search-query-string

No longer use the function `notmuch-search-get-query', which does
nothing but return the value of that variable.  That function was
added in [1: f47eeac0] for use in `notmuch-read-query' along-side
related `notmuch-show-get-query' and `notmuch-tree-get-query' but
using it here makes little sense.

1: f47eeac0b0186c3559eb559c4f0bee0e1fac1961
   emacs: set default in notmuch-read-query