Reline does not support convert-meta

[tompng]

When encoding is set to 7bit-encoding or convert-meta is turned on in inputrc, Readline converts byte(>= 128) to ESC + (byte & 0x7f).
Looking into the source code, it looks like Reline is trying to support it, but actually it does not.

We can either support it (if an environment using 8th bit as meta still exist), or clean up unused code.

How to reproduce

Bytes of Japanese letter あ is [227, 129, 130] which will be converted to ESC c ESC \C-a ESC \C-b.
To reproduce, type あ directly from IME or paste あ without using bracketed paste.

# ~/.inputrc
set enable-bracketed-paste off

Readline

LANG="" irb --readline
irb(main):001> Readline::VERSION
=> "8.2" # Make sure readline-ext is installed
# input "foobar\C-aあ"
irb(main):002> Foobar

Reline

LANG="" irb
irb(main):001> Encoding.default_external
=> #<Encoding:US-ASCII>
# input "foobar\C-aあ"
irb(main):002> foobar
# does not respond to keystrokes(except arrow keys)

[tompng]

#715 will remove code from that looks like trying to support convert-meta.
#723 will let reline work well configuring meta key binding without implementing convert-meta.