Revert "Add support for ssh-ed25519 to lib/net/ssh/buffer.rb"

This reverts commit 16398e414cdcf59939c31357c136a188fd10ecab.
Merge pull request #185 from mfazekas/ed25519_is_not_implemented

Revert "Add support for ssh-ed25519 to lib/net/ssh/buffer.rb"