cpu: Remove unnecessary noop methods

In the previous commits we made cpu_exec_* and debug_excp_handler
optional, so we can now remove these no-op handlers.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Claudio Fontana <cfontana@suse.de>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20201212155530.23098-13-cfontana@suse.de>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>