Revert "[microTVM] Add open and close methods to Session class"

This reverts commit 5976550.

Instead of adding an open() method to the API, contextlib.ExitStack() +
enter_context() will be used to manage the context for micro session.

python/tvm/micro/session.py

@@ -73,7 +73,7 @@ def __init__(
         ----------
         transport_context_manager : ContextManager[transport.Transport]
             If given, `flasher` and `binary` should not be given. On entry, this context manager
-            should establish a transport between this TVM instance and the device.
+            should establish a tarnsport between this TVM instance and the device.
         session_name : str
             Name of the session, used for debugging.
         timeout_override : TransportTimeouts
@@ -107,7 +107,7 @@ def _wrap_transport_write(self, data, timeout_microsec):
 
         return len(data)  # TODO(areusch): delete
 
-    def open(self):
+    def __enter__(self):
         """Initialize this session and establish an RPC session with the on-device RPC server.
 
         Returns
@@ -142,12 +142,6 @@ def open(self):
             self.transport.__exit__(*sys.exc_info())
             raise
 
-    def close(self):
-        self.transport.__exit__()
-
-    def __enter__(self):
-        return self.open()
-
     def __exit__(self, exc_type, exc_value, exc_traceback):
         """Tear down this session and associated RPC session resources."""
         if not self._exit_called: