Remove the CLIENTVER 005 token. (#161)

Remove the CLIENTVER 005 token. (#161)

This was introduced in commit bde6442c47 but the rationale for it is
pretty shaky. No other non-Charybdis-derived servers send it and the
features the original commit claims it can be used to detect all have
their own methods of detection. The concept of "core capabilities" and
versioned releases was also dropped by IRCv3 many years ago in favour
of living specifications.