layer.conf: Remove references to i2s-mmap
layer.conf: Remove references to i2s-mmap

This overlay is gone in Linux 4.9 which Pyro uses for all machine
types.

Signed-off-by: Will Newton <9ae5e71ad61fb3eb3f972773b758e0d4f6c5e573@resin.io>

resin-os/resin-raspberrypi,resin-os/resin-raspberrypi