ruby@1.8: delete formula.

Don't need this as Mavericks (the oldest version that will run Homebrew
as of 2.0.0) ships with Ruby 2.0.

Formula/ruby@1.8.rb

@@ -1,60 +0,0 @@
-# This formula should never be deleted even when it is in violation of
-# https://docs.brew.sh/Versions. This is because it is useful to test things
-# with Ruby 1.8 for reproducing Ruby issues with older versions of macOS that
-# used this version (e.g. on <=10.9 the system Ruby is 1.8 and we build our
-# portable Ruby on 10.5).
-
-class RubyAT18 < Formula
-  desc "Powerful, clean, object-oriented scripting language"
-  homepage "https://www.ruby-lang.org/"
-  url "https://cache.ruby-lang.org/pub/ruby/1.8/ruby-1.8.7-p374.tar.bz2"
-  sha256 "b4e34703137f7bfb8761c4ea474f7438d6ccf440b3d35f39cc5e4d4e239c07e3"
-  revision 5
-
-  bottle do
-    sha256 "7c6d46159711bc9024aac359d3bf1434bc190939d44573c3084c68b4060c9f36" => :mojave
-    sha256 "1a4f800a8db04522cefca4019d9100d7745e66db6caf253641a82bfe7cf9422c" => :high_sierra
-    sha256 "491e3b677ebb4fc688f6453457c3f540f7854c5fc13f34f42a8c25fb278b7d79" => :sierra
-  end
-
-  keg_only :versioned_formula
-
-  depends_on "pkg-config" => :build
-  depends_on "openssl"
-  depends_on "readline"
-
-  def install
-    # Compilation with `superenv` breaks because the Ruby build system sets
-    # `RUBYLIB` and `RUBYOPT` and this disturbs the wrappers for `cc` and
-    # `clang` that are provided by Homebrew and are implemented in Ruby. We
-    # prefix those invocations with `env` to clean the environment for us.
-    scrub_env = "/usr/bin/env RUBYLIB= RUBYOPT="
-    inreplace "mkconfig.rb", "    if /^prefix$/ =~ name\n",
-              <<~EOS.gsub(/^/, "    ")
-                # `env` removes stuff that will break `superenv`.
-                if %w[CC CPP LDSHARED LIBRUBY_LDSHARED].include?(name)
-                  val = val.sub("\\"", "\\"#{scrub_env} ")
-                end
-                if /^prefix$/ =~ name
-              EOS
-
-    rm_r "ext/tk"
-
-    args = %W[
-      --prefix=#{prefix}
-      --enable-shared
-      --enable-install-doc
-    ]
-
-    system "./configure", *args
-    system "make"
-    system "make", "install"
-    system "make", "install-doc"
-  end
-
-  test do
-    hello_text = shell_output("#{bin}/ruby -e 'puts :hello'")
-    assert_equal "hello\n", hello_text
-    system "#{bin}/ruby", "-e", "require 'zlib'"
-  end
-end