etc: Drop xtables.conf

etc: Drop xtables.conf

The file is not used since the commit this one fixes. Also it wasn't
installed until recently, when commit 3822a992bc277 ("Makefile: Fix for
'make distcheck'") added it in the wrong spot in an attempt to reduce
differences between tarballs generated by 'make tarball' and 'make
dist'.

While being at it, drop stale xtables_config_main() prototype from
xtables-multi.h.

Fixes: 06fd5e46d46f7 ("xtables: Drop support for /etc/xtables.conf")
Signed-off-by: Phil Sutter <phil@nwl.cc>