/var/ipfire/ethernet/settings: Drop BROADCAST variable

/var/ipfire/ethernet/settings: Drop BROADCAST variable

This variable is no longer being used and was only used to
assign IP addresses to the individual interfaces.

However, the kernel knows best which IP address to select
as broadcast address for each network. Therefore we depend
on the kernel which allows us to support RFC3021.

Fixes: #12486 - no /31 transfer net available on red
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>