netfilter: nf_tables: remove check against removal of inactive objects

The following sequence inside a batch, although not very useful, is
valid:

 add table foo
 ...
 delete table foo

This may be generated by some robot while applying some incremental
upgrade, so remove the defensive checks against this.

This patch keeps the check on the get/dump path by now, we have to
replace the inactive flag by introducing object generations.

Reported-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>