ir: remove redundant cycle detection code in equals

ir: remove redundant cycle detection code in equals

While looking at something else, I realized that in the overload for
the equals() function for class_or_union, the cycle detection
management code was redundant for the case where we are looking
comparing a decl-only class to another class.

This patch removes that redundant code.

	* src/abg-ir.cc (equals):  In the overload for class_or_union
	remove redundant cycle detection code when comparison a decl-only
	class to another class.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>