Revert "day2/vhdl: workaround ghdl#1529"

This reverts commit a73387e367.

day2/vhdl/parser.vhd

@@ -30,7 +30,7 @@ architecture behaviour of parser is
 
 	function char_to_digit(input : in character) return digit is
 	begin
-		if not (input < '0') and input <= '9' then
+		if input >= '0' and input <= '9' then
 			return character'pos(input) - character'pos('0');
 		else
 			return 0;