[ExecuTorch] Clean up pre-C++17 macros

We require C++17.

Differential Revision: [D65233922](https://our.internmc.facebook.com/intern/diff/D65233922/)

[ghstack-poisoned]

runtime/platform/compiler.h

@@ -75,27 +75,13 @@
 
 #endif // defined(__GNUC__)
 
-#if (__cplusplus) >= 201703L
-
 #define ET_DEPRECATED [[deprecated]]
 #define ET_EXPERIMENTAL \
   [[deprecated("This API is experimental and may change without notice.")]]
 #define ET_FALLTHROUGH [[fallthrough]]
 #define ET_NODISCARD [[nodiscard]]
 #define ET_UNUSED [[maybe_unused]]
 
-#else
-
-#define ET_DEPRECATED __attribute__((deprecated))
-#define ET_EXPERIMENTAL \
-  __attribute__((       \
-      deprecated("This API is experimental and may change without notice.")))
-#define ET_FALLTHROUGH __attribute__((fallthrough))
-#define ET_NODISCARD __attribute__((warn_unused_result))
-#define ET_UNUSED __attribute__((unused))
-
-#endif // (__cplusplus) >= 201703L
-
 // UNLIKELY Macro
 // example
 // if ET_UNLIKELY(a > 10 && b < 5) {