There was a problem in the iOS app where a callback was done passing a
registered mpPollClosure that pointed to the mainKit variable in
lokit_main() even though that variable had already gone out of scope
and been destructed.

Introduce unregisterPollCallbacks(), which just sets the mpPollClosure
pointer to null. That means no callbacks should be done any more. It
does not mean that Unipoll would not be active. Change isUnipoll() to
look at whether the mpPollCallback pointer has been set or not.

Change-Id: I5d5527c0ef097682679371dc642f8896ff05450d
Reviewed-on: https://gerrit.libreoffice.org/72283Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>