Bug 1820168 - Remove unused method -[nsCursorManager dispose]. r=mac-reviewers,bradwerth

Differential Revision: https://phabricator.services.mozilla.com/D172483

widget/cocoa/nsCursorManager.h

@@ -26,7 +26,6 @@
 - (void)setNonCustomCursor:(const nsIWidget::Cursor&)aCursor;
 
 + (nsCursorManager*)sharedInstance;
-+ (void)dispose;
 @end
 
 @interface NSCursor (Undocumented)

widget/cocoa/nsCursorManager.mm

@@ -37,11 +37,6 @@ static constexpr nsCursor kCustomCursor = eCursorCount;
   return gInstance;
 }
 
-+ (void)dispose {
-  [gInstance release];
-  gInstance = nil;
-}
-
 + (NSCursor*)freshCursorWithType:(enum nsCursor)aCursor {
   switch (aCursor) {
     case eCursor_standard: