Remove unused Incr ++ and Decr ++

[hachi8833]

While fixing #526, I found that token.Incr and token.Decr are used in lexer.go while increment/decrement operators are unused.

// compiler/token/token.go
	Incr     = "++"
	Decr     = "--"

I'd like to remove them when fixing #526. If @st0012 has any plan to use them, please let me know.

[st0012]

@hachi8833 They will not be used again, plz drop them, thx