xtables: Remove target_maxnamelen field

xtables: Remove target_maxnamelen field

This is a partial revert of commit 9f075031a1973 ("Combine
parse_target() and command_jump() implementations"): Upstream prefers to
reduce max chain name length of arptables by two characters instead of
the introduced struct xtables_globals field which requires to bump
library API version.

Fixes: 9f075031a1973 ("Combine parse_target() and command_jump() implementations")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>