Kill python binary path existince checking.

Since this is now done in PythonBinary internally, we can revert to the
cleaner check. Additionally, this self-contained handling now removes
the need for PythonInterpreter cache clearing in
test_select_interpreter.py.

src/python/pants/backend/python/tasks/select_interpreter.py

@@ -101,12 +101,6 @@ def _get_interpreter(self, interpreter_path_file, targets):
     if os.path.exists(interpreter_path_file):
       with open(interpreter_path_file, 'r') as infile:
         binary = infile.read().strip()
-        # TODO: Remove this temporary check once we have https://github.com/pantsbuild/pex/pull/886.
-        if not os.path.exists(binary):
-          self.context.log.info('Stale interpreter reference detected: {}, removing reference and '
-                                'selecting a new interpreter.'.format(binary))
-          os.remove(interpreter_path_file)
-          return self._select_interpreter(interpreter_path_file, targets)
       try:
         return PythonInterpreter.from_binary(binary)
       except PythonInterpreter.Error:

tests/python/pants_test/backend/python/tasks/test_select_interpreter.py

@@ -8,7 +8,6 @@
 from typing import Tuple
 
 from pex.interpreter import PythonInterpreter
-from pex.variables import ENV
 
 from pants.backend.python.interpreter_cache import PythonInterpreterCache
 from pants.backend.python.targets.python_library import PythonLibrary
@@ -18,7 +17,7 @@
 from pants.option.ranked_value import RankedValue
 from pants.python.python_setup import PythonSetup
 from pants.testutil.task_test_base import TaskTestBase
-from pants.util.dirutil import chmod_plus_x, safe_mkdtemp, safe_rmtree
+from pants.util.dirutil import chmod_plus_x, safe_mkdtemp
 
 
 class SelectInterpreterTest(TaskTestBase):
@@ -33,9 +32,6 @@ def setUp(self):
     # only one value no matter what arguments, environment or input stream it has attached. That
     # value is the interpreter identity which is a JSON dict with exactly the following keys:
     # binary, python_tag, abi_tag, platform_tag, version, supported_tags, env_markers.
-
-    # <impl> <abi> <impl_version> <major> <minor> <patch>
-
     def fake_interpreter(python_tag: str, abi_tag: str, version: Tuple[int, int, int]):
       interpreter_dir = safe_mkdtemp()
       binary = os.path.join(interpreter_dir, 'python')
@@ -90,11 +86,6 @@ def _fake_target(self, spec, compatibility=None, sources=None, dependencies=None
                             dependencies=dependencies, compatibility=compatibility)
 
   def _select_interpreter(self, target_roots, should_invalidate=None):
-    # TODO: Call PythonInterpreter.clear_cache() instead, when
-    # https://github.com/pantsbuild/pex/pull/885 is available.
-    interpreter_cache_dir = os.path.join(ENV.PEX_ROOT, 'interpreters')
-    safe_rmtree(interpreter_cache_dir)
-
     context = self.context(target_roots=target_roots)
 
     task = self.create_task(context)