Remove observers on dealloc (signalapp#1379)

// FREEBIE

Author: michaelkirk

Signal/src/views/OWSExpirationTimerView.m

@@ -24,6 +24,11 @@ @interface OWSExpirationTimerView ()
 
 @implementation OWSExpirationTimerView
 
+- (void)dealloc
+{
+    [[NSNotificationCenter defaultCenter] removeObserver:self];
+}
+
 - (instancetype)initWithFrame:(CGRect)frame
 {
     self = [super initWithFrame:frame];