configure: remove obsoleted $sparc_cpu variable

9b9c37c3 ("tcg-sparc: Assume v9 cpu always, i.e. force v8plus in
32-bit mode.", 2012-09-21) removed the need for this variable and
most of the references to it, but this one.

Remove defunct code, no effect or functionality change expected.

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190709234330.89699-1-carenas@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>