MINOR: server: extract unregistering from "delete server" CLI parser

MINOR: server: extract unregistering from "delete server" CLI parser

Extract the code which performs the server deletion from the "del
server" CLI parser in a new function named srv_unregister(). Its purpose
is to ensure that a server is detached from any visible elements, most
notably the parent proxy list and name tree, and the global GUID tree.
This step is mandatory to remove a server at runtime but is skipped on
deinit.

This reorg allows to implement alternative method for server deletion at
runtime. This will be useful for the future "purge server" CLI command.