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/misc/hashtable.hpp

@@ -239,8 +239,7 @@ public:
 	/** non-const item search & removal */
 	void Pop(Titem_ &item)
 	{
-		bool ret = TryPop(item);
-		(void)ret; // assert only
+		[[maybe_unused]] bool ret = TryPop(item);
 		assert(ret);
 	}