macosx: Workaround crash in PXSourceList dealloc

[super setDataSource] crashes on macOS 10.11 until 10.13.
The code in question should not be needed (the recieving
properties are weak in new macOS frameworks).
Also whole deallocation of this object is not consistent.
Therefore remove the code in question.

Closes videolan/vlc#21267

modules/gui/macosx/PXSourceList/PXSourceList.m

@@ -67,14 +67,6 @@ NSString * const PXSLDeleteKeyPressedOnRowsNotification = @"PXSourceListDeleteKe
     _delegateDataSourceProxy = [[PXSourceListDelegateDataSourceProxy alloc] initWithSourceList:self];
 }
 
-- (void)dealloc
-{
-    _delegateDataSourceProxy = nil;
-    //Remove ourselves as the delegate and data source to be safe
-    [super setDataSource:nil];
-    [super setDelegate:nil];
-}
-
 
 #pragma mark -
 #pragma mark Custom Accessors