Only try to remove hook from sys.meta_path if it's present

Prevent error on exit if some code messes with sys.meta_path and removes the
assertionrewrite hook (CaptureMock seems to do this):

      File "/Users/marca/dev/hg-repos/pytest/_pytest/assertion/__init__.py", line 64, in pytest_unconfigure
        sys.meta_path.remove(hook)
    ValueError: list.remove(x): x not in list

--HG--
branch : sys_meta_path_remove_hook_only_if_present

_pytest/assertion/__init__.py

@@ -60,7 +60,7 @@ def pytest_configure(config):
 
 def pytest_unconfigure(config):
     hook = config._assertstate.hook
-    if hook is not None:
+    if hook is not None and hook in sys.meta_path:
         sys.meta_path.remove(hook)
 
 def pytest_collection(session):

testing/test_assertrewrite.py

@@ -482,6 +482,22 @@ def test_rewritten():
     assert "@py_builtins" in globals()""".replace("\n", "\r\n"), "wb")
         assert testdir.runpytest().ret == 0
 
+    def test_sys_meta_path_munged(self, testdir):
+        # In some versions, if any code messed with sys.meta_path and removed
+        # the assertionrewrite import hook (one example is CaptureMock), it
+        # would cause an error on py.test exit:
+        #
+        #   File "/Users/marca/dev/hg-repos/pytest/_pytest/assertion/__init__.py", line 64, in pytest_unconfigure
+        #     sys.meta_path.remove(hook)
+        # ValueError: list.remove(x): x not in list
+        #
+        testdir.tmpdir.join("test_meta_path.py").write("""#!/usr/bin/env python
+def test_meta_path():
+    import sys
+    sys.meta_path = []
+    assert True""".replace("\n", "\r\n"), "wb")
+        assert testdir.runpytest().ret == 0
+
     def test_write_pyc(self, testdir, tmpdir, monkeypatch):
         from _pytest.assertion.rewrite import _write_pyc
         from _pytest.assertion import AssertionState