worker: remove Python 2 support code

Differential Revision: https://phab.mercurial-scm.org/D12304

mercurial/worker.py

@@ -64,11 +64,10 b' def _numworkers(ui):'
     return min(max(countcpus(), 4), 32)
 
 
-if pycompat.ispy3:
-
     def ismainthread():
         return threading.current_thread() == threading.main_thread()
 
+
     class _blockingreader(object):
         def __init__(self, wrapped):
             self._wrapped = wrapped
@@ -100,17 +99,6 b' if pycompat.ispy3:'
             return bytes(buf)
 
 
-else:
-
-    def ismainthread():
-        # pytype: disable=module-attr
-        return isinstance(threading.current_thread(), threading._MainThread)
-        # pytype: enable=module-attr
-
-    def _blockingreader(wrapped):
-        return wrapped
-
-
 if pycompat.isposix or pycompat.iswindows:
     _STARTUP_COST = 0.01
     # The Windows worker is thread based. If tasks are CPU bound, threads