Codechange: use "[[maybe_unused]]" instead of a wide variety of other ways we had

While at it, replace OTTD_ASSERT with WITH_ASSERT, as this
is always set if assert() is valid. No matter if NDEBUG is set
or not.

src/stdafx.h

@@ -435,11 +435,6 @@ void NORETURN CDECL error(const char *str, ...) WARN_FORMAT(1, 2);
 #	define assert(expression) if (unlikely(!(expression))) error("Assertion failed at line %i of %s: %s", __LINE__, __FILE__, #expression);
 #endif
 
-/* Asserts are enabled if NDEBUG isn't defined or WITH_ASSERT is defined. */
-#if !defined(NDEBUG) || defined(WITH_ASSERT)
-#	define OTTD_ASSERT
-#endif
-
 #if defined(OPENBSD)
 	/* OpenBSD uses strcasecmp(3) */
 #	define _stricmp strcasecmp