As replaced the generic CONFIG_SH4 by more fine-grained
selectors, we can remove this now unused config variable.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210222141514.2646278-9-f4bug@amsat.org>