Ignore can_fail attribute in Neovim functions

Starting with nvim 0.1.6 the can_fail attribute is no longer part of
the API metadata. This commit removes the can_fail check in the
comparison operator.

src/function.cpp

@@ -75,10 +75,6 @@ bool Function::operator==(const Function& other)
 		return false;
 	}
 
-	if ( this->can_fail != other.can_fail ) {
-		return false;
-	}
-
 	if ( this->return_type != other.return_type ) {
 		return false;
 	}