MINOR: stats: get rid of the STAT_SHOWADMIN flag

MINOR: stats: get rid of the STAT_SHOWADMIN flag

This flag is used to decide to show the check box in front of a proxy
on the HTML stat page. It is always equal to STAT_ADMIN except when the
proxy has no backend capability (i.e. a pure frontend) or has no server,
in which case it's only used to avoid leaving an empty column at the
beginning of the table. Not only this is pretty useless, but it also
causes the columns not to align well when mixing multiple proxies with
or without servers.

Let's simply always use STAT_ADMIN and get rid of this flag.