Remove instance variable @first_char

[tompng]

When Reline reads EOF, Reline.readline should return nil if and only if input is empty.

Readline

$ printf "a\b" | ruby -rreadline -e "p Readline.readline"
nil
$ printf "" | ruby -rreadline -e "p Readline.readline" 
nil
$ printf "a" | ruby -rreadline -e "p Readline.readline" 
a
"a"

Reline

$ printf "" | ruby -Ilib -rreline -e "p Reline.readline" 
nil
$ printf "a\b" | ruby -Ilib -rreline -e "p Reline.readline"
nil # it was ""

We don't need @first_char. We just need current_input.empty? check.