DRIVERS.md: remove outdated refs to IRQ_EVENT.

Author: peterhinch

v3/docs/DRIVERS.md

@@ -40,7 +40,6 @@ Drivers are imported with:
 from primitives.switch import Switch
 from primitives.pushbutton import Pushbutton
 from primitives.aadc import AADC
-from primitives.irq_event import IRQ_EVENT
 ```
 There is a test/demo program for the Switch and Pushbutton classes. On import
 this lists available tests. It assumes a Pyboard with a switch or pushbutton
@@ -55,12 +54,6 @@ is run as follows:
 from primitives.tests.adctest import test
 test()
 ```
-The test for the `IRQ_EVENT` class requires a Pyboard with pins X1 and X2
-linked. It is run as follows:
-```python
-from primitives.tests.irq_event_test import test
-test()
-```
 
 ###### [Contents](./DRIVERS.md#1-contents)