www/firefox: fix running on wayland

since the latest version firefox uses a wayland-proxy to deal with
disconnections from the compositor, this proxy had portability issue
in particular it relies on a non defined behaviour polling a non
connected socket, remove the test on the polling to make it run on
FreeBSD

Note for users running into other issues with wayland-proxy they can for
now disable it via: MOZ_DISABLE_WAYLAND_PROXY=1 env var.

This has been reported upstream:
https://bugzilla.mozilla.org/show_bug.cgi?id=1882972
https://github.com/stransky/wayland-proxy/issues/3
https://github.com/stransky/wayland-proxy/pull/6

Reviewd by: manu
Tested by: manu