MINOR: server: add helper function to detach server from proxy list

MINOR: server: add helper function to detach server from proxy list

Remove some code duplication by introducing a basic helper function
to detach a server from its parent proxy. It is supported to call
the function even if the server is not yet listed in the proxy list.

If the server is not yet listed in the proxy, the function will do
nothing. In delete_server(), we previously performed some BUG_ON()
to ensure that the detach always succeeded given that we were certain
that the server was in the proxy list because it was retrieved through
get_backend_server().

However this test is superfluous, we can safely assume that the operation
will always succeed if get_backend_server() returned != NULL (we're under
full thread isolation), and if it's not the case, then we have a bigger
API issue anyway..