regmap-irq: Cleanups and remove unused

Merge series from Aidan MacDonald <aidanmacdonald.0x0@gmail.com>:

This is a straightforward patch series, mostly just removing a bunch
of old features that were only used by a handful of drivers.

- 1/4 and 2/4 remove unused, deprecated functionality
- 3/4 makes the behavior of .handle_mask_sync() a bit more consistent
  w.r.t. mask and unmask registers, to aid maintainability.
- 4/4 removes now-unused "inverted mask/unmask" compatibility code.