Fix multiwaiter public header.

[davidchisnall]

It had a load of left-over things in it from when the scheduler knew about event groups.

[nwf]

With the test caught up, it LGTM. Please squash, not rebase, merge for ease of any future bisections.

[nwf]

(There's a bit of cruft, like

cheriot-rtos/sdk/core/scheduler/multiwait.h

Line 36 in bea973d

unsigned flags : 6 = 0;

and the fact that ::readyEvents is used as a 24-bit boolean, that could also be cleaned up, or left in case we ever wanted something like it again. I think it's not enough cruft that removing it would save bytes in an EventWaiter object?)