Remove deprecated 'pytest_itemstart' hook

This hook has been deprecated/removed for more than 10 years
in a2fe6714f8.

changelog/6637.breaking.rst

@@ -0,0 +1,3 @@
+Removed the long-deprecated ``pytest_itemstart`` hook.
+
+This hook has been marked as deprecated and not been even called by pytest for over 10 years now.

src/_pytest/hookspec.py

@@ -307,10 +307,6 @@ def pytest_runtestloop(session):
     """
 
 
-def pytest_itemstart(item, node):
-    """(**Deprecated**) use pytest_runtest_logstart. """
-
-
 @hookspec(firstresult=True)
 def pytest_runtest_protocol(item, nextitem):
     """ implements the runtest_setup/call/teardown protocol for