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/gfxinit.cpp

@@ -101,8 +101,7 @@ static void LoadGrfFileIndexed(const char *filename, const SpriteID *index_tbl,
 		uint end = *index_tbl++;
 
 		do {
-			bool b = LoadNextSprite(start, file, sprite_id);
-			(void)b; // Unused without asserts
+			[[maybe_unused]] bool b = LoadNextSprite(start, file, sprite_id);
 			assert(b);
 			sprite_id++;
 		} while (++start <= end);