Remove old mentions and uses of ipython_extension_dir (#14310)

The bulk of this functionality was removed in IPython 8.6.0, but there
were still mentions of it in the docs and in the reload code.

The extant mentions of `.ipython/extensions` in code came up as a source
of confusion in  #14263.

IPython/core/extensions.py

@@ -42,28 +42,15 b' class ExtensionManager(Configurable):'
     if the extension is reloaded.
 
     You can put your extension modules anywhere you want, as long as
-    they can be imported by Python's standard import mechanism.  However,
-    to make it easy to write extensions, you can also put your extensions
-    in ``os.path.join(self.ipython_dir, 'extensions')``.  This directory
-    is added to ``sys.path`` automatically.
+    they can be imported by Python's standard import mechanism.
     """
 
     shell = Instance('IPython.core.interactiveshell.InteractiveShellABC', allow_none=True)
 
     def __init__(self, shell=None, **kwargs):
         super(ExtensionManager, self).__init__(shell=shell, **kwargs)
-        self.shell.observe(
-            self._on_ipython_dir_changed, names=('ipython_dir',)
-        )
         self.loaded = set()
 
-    @property
-    def ipython_extension_dir(self):
-        return os.path.join(self.shell.ipython_dir, u'extensions')
-
-    def _on_ipython_dir_changed(self, change):
-        ensure_dir_exists(self.ipython_extension_dir)
-
     def load_extension(self, module_str: str):
         """Load an IPython extension by its module name.
 
@@ -124,7 +111,6 b' class ExtensionManager(Configurable):'
         :func:`reload` is called and then the :func:`load_ipython_extension`
         function of the module, if it exists is called.
         """
-        from IPython.utils.syspathcontext import prepended_to_syspath
 
         if BUILTINS_EXTS.get(module_str, False) is True:
             module_str = "IPython.extensions." + module_str
@@ -132,7 +118,6 b' class ExtensionManager(Configurable):'
         if (module_str in self.loaded) and (module_str in sys.modules):
             self.unload_extension(module_str)
             mod = sys.modules[module_str]
-            with prepended_to_syspath(self.ipython_extension_dir):
                 reload(mod)
             if self._call_load_ipython_extension(mod):
                 self.loaded.add(module_str)