remove non-single-threaded option

Single-threaded is the build-default, and seems to work well enough for
1.12.7, so remove the option to turn it off.

This is because build-options that influence such low-level/core
behavior are a pain to maintain.

mu4e/meson.build

@@ -1,4 +1,4 @@
-## Copyright (C) 2022 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+## Copyright (C) 2022-2024 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
 ##
 ## This program is free software; you can redistribute it and/or modify
 ## it under the terms of the GNU General Public License as published by
@@ -17,9 +17,6 @@
 
 # generate some build data for use in mu4e
 version_extra=''
-if get_option('xapian-single-threaded')
-  version_extra='-st'
-endif
 
 mu4e_meta = configure_file(
   input: 'mu4e-config.el.in',

mu4e/mu4e-main.el

@@ -299,9 +299,7 @@ Otherwise, do nothing."
          "* "
          (propertize "mu4e" 'face 'mu4e-header-key-face)
          (propertize " - mu for emacs version " 'face 'mu4e-title-face)
-         (propertize  (concat mu4e-mu-version
-                              (if (mu4e--server-xapian-single-threaded-p) "-st" ""))
-                      'face 'mu4e-header-key-face)
+         (propertize  mu4e-mu-version 'face 'mu4e-header-key-face)
          "\n\n"
          (propertize "  Basics\n\n" 'face 'mu4e-title-face)
          (mu4e--main-action