[router-adv] have RouterAdvMessage inherit from Unequatable (#6580)

src/core/border_router/router_advertisement.hpp

@@ -47,6 +47,7 @@
 #include <openthread/platform/toolchain.h>
 
 #include "common/encoding.hpp"
+#include "common/equatable.hpp"
 #include "net/icmp6.hpp"
 #include "net/ip6.hpp"
 
@@ -386,7 +387,7 @@ static_assert(sizeof(RouteInfoOption) == 24, "invalid RouteInfoOption structure"
  *
  */
 OT_TOOL_PACKED_BEGIN
-class RouterAdvMessage
+class RouterAdvMessage : public Unequatable<RouterAdvMessage>
 {
 public:
     /**
@@ -459,18 +460,6 @@ public:
      */
     bool operator==(const RouterAdvMessage &aOther) const;
 
-    /**
-     * This method overloads operator `!=` to evaluate whether or not
-     * two instances of `RouterAdvMessage` are equal.
-     *
-     * @param[in]  aOther  The other `RouterAdvMessage` instance to compare with.
-     *
-     * @retval TRUE   If the two `RouterAdvMessage` instances are not equal.
-     * @retval FALSE  If the two `RouterAdvMessage` instances are equal.
-     *
-     */
-    bool operator!=(const RouterAdvMessage &aOther) const { return !(*this == aOther); }
-
 private:
     enum : uint8_t
     {