firewall3: remove unnecessary fw3_has_table

Given that firewall3 already skips the table when fw3_ipt_open fails,
there is no need for fw3_has_table.

Furthermore, /proc/net/ip_tables_names is not reliable under linux
containers (e.g. Docker/LXC/LXD). This patch will remove the need for
existing hacks required for OpenWrt to run on those platforms.

Reviewed-by: Rui Salvaterra <rsalvaterra@gmail.com>
Tested-by: Rui Salvaterra <rsalvaterra@gmail.com>
Tested-by: Ansuel Smith <ansuelsmth@gmail.com>
Signed-off-by: Wenli Looi <wlooi@ucalgary.ca>