[NFC][libc++][test] Removes unneeded code.

[Mordante]

Summary

In D130295 @mumbleskates wondered why std::strong_ordering::equal had
special code since it's the same as std::strong_ordering::equivalent.

This is indeed the case so the special case can be removed.

[mumbleskates]

Thanks for looking!

libcxx/test/support/test_comparisons.h

 constexpr void AssertOrderReturn() {
     AssertComparisonsReturnBool<T, U>();
     ASSERT_SAME_TYPE(decltype(std::declval<const T&>() <=> std::declval<const U&>()), Order);
 }
 
 template <class Order, class T, class U = T>
 constexpr bool testOrder(const T& t1, const U& t2, Order order) {
     bool equal = order == Order::equivalent;
-    if constexpr (std::same_as<Order, std::strong_ordering>)
-        equal |= order == Order::equal;
-
     bool less = order == Order::less;
 
     return (t1 <=> t2 == order) && testComparisons(t1, t2, equal, less);
 }
 
 template <class T, class Param>
 constexpr bool testOrderValues(Param val1, Param val2) {
   return testOrder(T(val1), T(val2), val1 <=> val2);