Revert "Package-Requires: add emacs-23.2"

This reverts commit 3f2f81b347bff884ad4759ed4c8b12c204ce75e0.

No version of `package.el' which works with Emacs 23.x
understands the "emacs" virtual package.  See issue #1369.
magit-read-gpg-secret-key: don't require match

Let the user exit with any input.  This allows selecting a subkey,
even though we don't offer such keys for completion.