docs: Note 'config' object shouldn't be accessed after initial load

Update Code_Overview.md to note that the config object should not be
stored after the "config loading phase".

Remove a few inadvertent cases where a 'config' object was stored
in module member variables.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/extras/ads1x1x.py

@@ -321,7 +321,6 @@ class ADS1X1X_pin:
     def __init__(self, chip, config):
         self.mcu = chip.mcu
         self.chip = chip
-        self.config = config
 
         self.invalid_count = 0