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_options.txt

@@ -48,7 +48,6 @@ option('guile-extension-dir',
        type: 'string',
        description: 'custom install path for the guile extension module')
 
-
 #
 # misc
 #
@@ -58,11 +57,6 @@ option('tests',
        value: 'auto',
        description: 'build unit tests')
 
-option('xapian-single-threaded',
-       type : 'boolean',
-       value: true,
-       description: 'only use Xapian from a single thread')
-
 option('readline',
        type: 'feature',
        value: 'auto',