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.

lib/tests/bench-indexer.cc

@@ -467,7 +467,6 @@ benchmark_indexer(gconstpointer testdata)
 		auto store{Store::make_new(BENCH_STORE, BENCH_MAILDIRS)};
 		g_assert_true(!!store);
 		Indexer::Config conf{};
-		conf.max_threads = tdata->num_threads;
 
 		auto res = store->indexer().start(conf);
 		g_assert_true(res);