MINOR: resolvers: Remove server from named_servers tree when removing a SRV item

MINOR: resolvers: Remove server from named_servers tree when removing a SRV item

When a server is cleaned up because the corresponding SRV item is removed,
we always remove the server from the srvrq's name_servers tree. For now, it
is useless because, if a server was attached to a SRV item, it means it was
already removed from the tree. But it will be mandatory to fix a bug.

(cherry picked from commit 73001ab6e39fb25ff406bbcd3809d0a1fa0131b5)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit a62d9743fd4efaab5e08c94229af390e4afcd17a)
[cf: Changes applied in src/dns.c instead of src/resolvers.c]
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>