MAINT-5232: Remove dubious polling in LLMuteList::getInstance().

The LLMuteList singleton instance might be requested before gMessageSystem is
constructed. LLMuteList wants to register a couple gMessageSystem callbacks.
Since gMessageSystem is not (yet) itself an LLSingleton, LLMuteList's
constructor can't just call it into existence. Until now, LLMuteList overrode
LLSingleton's getInstance() method: every time getInstance() was called, the
subclass override method would check whether gMessageSystem had been
initialized, and if so, register its callbacks before forwarding the call to
the base-class LLSingleton::getInstance() method.
Change to use LLPounceable::callWhenReady() instead.
This is the reason gMessageSystem was made an LLPounceable.