Switch : Remove bogus default arg on inputIndex() private method

The context is in fact required to be non-null.

include/Gaffer/Switch.h

@@ -109,7 +109,7 @@ class GAFFER_API Switch : public ComputeNode
 		void childAdded( GraphComponent *child );
 		void plugSet( Plug *plug );
 		void plugInputChanged( Plug *plug );
-		size_t inputIndex( const Context *context = nullptr ) const;
+		size_t inputIndex( const Context *context ) const;
 
 		// Returns the input corresponding to the output and vice versa. Returns null
 		// if plug is not meaningful to the switching process.