MINOR: server: do not return next server on srv_drop()

Previously, srv_drop() returned the next server entry in the parent
proxy. This was used as convenience during server iteration. However,
the code has evolved several times to better deal with the server
deletion risk. Currently, srv_drop() return value is only used in a
single place.

Thus, this patch simplifies srv_drop() by removing its return value. As
a side effect, this will also reduce the modification required to
convert the proxy server list to a doubly linked struct list.