remove unused CombinedLifeMeter::IsFailing. Nothing uses it, so

it probably doesn't work.  Maybe a cleaner way if this is needed would
be to have a stub regular LifeMeter that points to CombinedLifeMeter, so
we don't need repetitive logic.

stepmania/src/CombinedLifeMeter.h

@@ -19,7 +19,6 @@ public:
 	/* Change life after receiving a hold note grade.  tscore is the score
 	 * received for the initial tap note. */
 	virtual void ChangeLife( PlayerNumber pn, HoldNoteScore hns, TapNoteScore tns ) = 0;
-	virtual bool IsFailing( PlayerNumber pn ) = 0;
 	virtual void OnTaunt() {};
 };
 

stepmania/src/CombinedLifeMeterTug.h

@@ -15,7 +15,6 @@ public:
 
 	virtual void ChangeLife( PlayerNumber pn, TapNoteScore score );
 	virtual void ChangeLife( PlayerNumber pn, HoldNoteScore score, TapNoteScore tscore );
-	virtual bool IsFailing( PlayerNumber pn ) { return false; };
 
 protected:
 	void ChangeLife( PlayerNumber pn, float fPercentToMove );

stepmania/src/ScreenGameplay.cpp

@@ -1564,9 +1564,7 @@ void ScreenGameplay::Update( float fDeltaTime )
 	{
 		PlayerNumber pn = pi->GetStepsAndTrailIndex();
 
-		if(
-			(pi->m_pLifeMeter && pi->m_pLifeMeter->IsFailing()) || 
-			(m_pCombinedLifeMeter && m_pCombinedLifeMeter->IsFailing(pn)) )
+		if( pi->m_pLifeMeter && pi->m_pLifeMeter->IsFailing() )
 		{
 			pi->GetPlayerState()->m_HealthState = PlayerState::DEAD;
 		}
@@ -1603,8 +1601,6 @@ void ScreenGameplay::Update( float fDeltaTime )
 			// check for individual fail
 			if( pi->m_pLifeMeter == NULL || !pi->m_pLifeMeter->IsFailing() )
 				continue; /* isn't failing */
-			if( m_pCombinedLifeMeter && !m_pCombinedLifeMeter->IsFailing(pn) )
-				continue; /* isn't failing */
 			if( pi->GetPlayerStageStats()->bFailed )
 				continue; /* failed and is already dead */