Remove unused SynchronousExecutor and tsinfer/utils.py

SynchronousExecutor was never used in the main package, only tested.

Author: jeromekelleher

tests/test_ancestors.py

@@ -35,7 +35,7 @@
 import tskit
 import zarr
 
-from tsinfer import ancestors, config, utils, vcz
+from tsinfer import ancestors, config, vcz
 
 # ---------------------------------------------------------------------------
 # Python reference implementation of AncestorBuilder