cipher: remove Cipher#encrypt(password, iv) form

[rhenium]

OpenSSL::Cipher#encrypt and #decrypt have long supported a hidden feature to derive a key and an IV from the String argument, but in an inappropriate way.

This feature is undocumented, untested, and has been deprecated since Ruby trunk r6549 on 2004-06-30, which started printing a non-verbose warning. More than 20 years later, it must be safe to remove it entirely.

The deprecated usage:

# `password` is a String, `iv` is either a String or nil
cipher = OpenSSL::Cipher.new("aes-256-cbc")
cipher.encrypt(password, iv)
p cipher.update("data") << cipher.final

was equivalent to:

cipher = OpenSSL::Cipher.new("aes-256-cbc")
cipher.encrypt

iv ||= "OpenSSL for Ruby rulez!"
key = ((cipher.key_len + 15) / 16).times.inject([""]) { |ary, _|
  ary << OpenSSL::Digest.digest("MD5", ary.last + password + iv[0, 8].ljust(8, "\0"))
}.join
cipher.key = key[...cipher.key_len]
cipher.iv = iv[...cipher.iv_len].ljust(cipher.iv_len, "\0")
p cipher.update("data") << cipher.final