MINOR: filters: Add a function to remove a filter conf from all proxy lists

MINOR: filters: Add a function to remove a filter conf from all proxy lists

A legacy filter configuration is linked in three lists of its proxy: the
global order with the 'list' member and the request and response orders
with the 'by_req' and 'by_res' members. Removing such a configuration
meant unlinking the three members by hand, in every filter supporting an
implicit declaration replaced by an explicit one.

The new flt_conf_delete() helper does it at once, as the counterpart of
flt_conf_add() and flt_conf_insert(). It will be used by the filters
parsing an explicit declaration that replaces an implicit one.