Remove --classic build entirely

The Unix build was the last to retain the classic build scheme.  The
new unified scheme has matured enough, even though some details may
need polishing.

Reviewed-by: Rich Salz <rsalz@openssl.org>
diff --git a/Configurations/README b/Configurations/README
index a5a006e..a05e02c 100644
--- a/Configurations/README
+++ b/Configurations/README
@@ -99,8 +99,7 @@
                            some options.  In this case, the first
                            string in the list is the name of the build
                            scheme.
-                           Currently recognised build schemes are
-                           "unixmake" and "unified".
+                           Currently recognised build scheme is "unified".
                            For the "unified" build scheme, this item
                            *must* be an array with the first being the
                            word "unified" and the second being a word