ARM: dts: bcm27xx: Remove enable_headphones setting

The enable_headphones parameter of the snd_bcm2835 module is forced
to 1 if enable_compat_alsa is 0, so setting them both on the kernel
command line is pointless (and, in the case of Pi 400 and Pi Zeroes,
confusing).

Signed-off-by: Phil Elwell <phil@raspberrypi.com>