Remove read_adc_value / read_gpio_value (console commands 0x1E/0x1F retired) #302

Part of OpenwaterHealth/openmotion-console-fw#12 (parent): the console firmware never implemented OW_CTRL_READ_ADC (0x1E) / OW_CTRL_READ_GPIO (0x1F), and both are being retired.

SDK side: remove the two constants from omotion/config.py, MotionConsole.read_adc_value() / read_gpio_value(), their xfail tests in tests/test_console.py, and their entries in docs/TestPlan.md.

Status and discussion live on the parent.