CLEANUP: resolvers: do not export resolv_purge_resolution_answer_records()

CLEANUP: resolvers: do not export resolv_purge_resolution_answer_records()

This code is dangerous enough that we certainly don't want external code
to ever approach it, let's not export unnecessary functions like this one.
It was made static and a comment was added about its purpose.

(cherry picked from commit 2acc160c05c0c8dc28ecce5941451c916d29bc63)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit aab7593db39e1083a77ac792723623bd9c402273)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>