removeObserver:forKeyPath:context: is not available in iOS 4.3

Since the exact context is not necessary this method can safely
be replaced with removeObserver:forKeyPath:, which is available in
iOS 4.3. 

This is not necessary for the 3.5.x version of the SDK (since it drops 4.3 compatibility), but it would be nice to have this applied to the 3.2.x branch of the SDK.

https://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Protocols/NSKeyValueObserving_Protocol/Reference/Reference.html#//apple_ref/occ/instm/NSObject/removeObserver:forKeyPath:context:

Introduced in https://github.com/facebook/facebook-ios-sdk/pull/509