hw/i386/pc: Remove unused pc_compat_2_3 declarations

hw/i386/pc: Remove unused pc_compat_2_3 declarations

We removed the implementations in commit 46a2bd52571
("hw/i386/pc: Remove deprecated pc-i440fx-2.3 machine")
but forgot to remove the declarations. Do it now.

Fixes: 46a2bd52571 ("hw/i386/pc: Remove deprecated pc-i440fx-2.3 machine")
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>