Remove useless member of BackendParameters.

Oversight in e2562667, which stopped using SpinlockSemaArray but forgot
to remove it from the array.

Reported-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/310f4005-91d7-42b2-ac70-92624260dd28%40iki.fi

src/backend/postmaster/launch_backend.c

@@ -108,7 +108,6 @@ typedef struct
 #ifdef USE_INJECTION_POINTS
 	struct InjectionPointsCtl *ActiveInjectionPoints;
 #endif
-	PGSemaphore *SpinlockSemaArray;
 	int			NamedLWLockTrancheRequests;
 	NamedLWLockTranche *NamedLWLockTrancheArray;
 	LWLockPadded *MainLWLockArray;