MINOR: server: fix message report when IDRAIN is set and MAINT is cleared

MINOR: server: fix message report when IDRAIN is set and MAINT is cleared

Remaining in drain mode after removing one of server admins flags leads
to this message being generated:

"Server name/backend is leaving forced drain but remains in drain mode."

However this is not necessarily true: the server might just be leaving
MAINT with the IDRAIN flag set, so the report is incorrect in this case.
(FDRAIN was not set so it cannot be cleared)

To prevent confusion around this message and to comply with the code
comment above it: we remove the "leaving forced drain" precision to
make the report suitable for multiple transitions.