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.

meson.build

@@ -133,13 +133,6 @@ add_project_arguments(['-DHAVE_CONFIG_H'], language: 'cpp')
 config_h_dep=declare_dependency(
   include_directories: include_directories(['.']))
 
-#
-# single-threaded Xapian access?
-#
-if get_option('xapian-single-threaded')
-  config_h_data.set('XAPIAN_SINGLE_THREADED', true)
-  message('use Xapian only in a single thread')
-endif
 #
 # d_type, d_ino are not available universally, so let's check
 # (we use them for optimizations in mu-scanner