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/townname.cpp

@@ -600,9 +600,7 @@ static char *MakeCzechTownName(char *buf, const char *last, uint32 seed)
 		return strecpy(buf, _name_czech_real[SeedModChance(4, lengthof(_name_czech_real), seed)], last);
 	}
 
-#ifdef WITH_ASSERT
-	const char *orig = buf;
-#endif
+	[[maybe_unused]] const char *orig = buf;
 
 	/* Probability of prefixes/suffixes
 	 * 0..11 prefix, 12..13 prefix+suffix, 14..17 suffix, 18..31 nothing */