bgpd: Fix crash when showing filtered routes

bgpd: Fix crash when showing filtered routes

This commit fixes the issue mentioned in #2419, which is caused by a
double-free. The problem of the current implementation is that
*bgp_input_modifier* already frees the passed attributes under specific
circumstances, which can then lead to a double-free as *bgp_attr_undup*
does not check if the attributes are set to NULL.

As it is not transparent to the function caller if the attributes get
freed or not and the similar function *bgp_output_modifier* also does
not flush the passed attributes, the line has been removed altogether.

All callers of *bgp_input_modifier* already deal by themself with
freeing/flushing/unduping BGP attributes, so it is safe to remove.

Signed-off-by: Pascal Mathis <mail@pascalmathis.com>