[nfc] remove unused include

Author: kavon

lib/SILGen/SILGenExpr.cpp

@@ -15,7 +15,6 @@
 #include "Callee.h"
 #include "Condition.h"
 #include "Conversion.h"
-#include "ExitableFullExpr.h"
 #include "Initialization.h"
 #include "LValue.h"
 #include "RValue.h"