Revert "www/chromium: Change default audio output to PULSEAUDIO to make chromium to play nice with desktop environments"

This reverts commit 89e432ced730385869e5bf6df4b705facbd1c947.

Reason: Several people complained that the PulseAudio dependency is a lot heavier than Sndio.

This is currently a controversial issue because:

1. PulseAudio is indeed a heavier dependency compared to Sndio, so for people who prefer minimalistic setup Sndio should be a default.
2. Currently desktop environments work best with PulseAudio, so it is better for average users to have PulseAudio as a default backend.

It would be best if www/chromium would allow to change audio backend at-startup
or on-the-fly without the need to change port options.

This can be achieved by the following methods:

1. Flavorizing www/chromium based on audio backends.
2. Creating slave ports with different audio backends.
3. Implementing on-the-fly or at-startup setting of the audio backend.

In https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=246449#c5 Robert Nagy <rnagy@FreeBSD.org>
volunteered to implement audio backend dependency in a proper way.

I've suggested that the item 3. above is a proper way for chromium.

PR: 246449