tcg: Remove TCG_TARGET_CON_STR_H

All backends have now been converted to tcg-target-con-str.h,
so we can remove the fallback code.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>