tcg: Remove redundant tcg_target_init checks

We've got a compile-time check for the condition in exec/cpu-defs.h.

Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: liguang <lig.fnst@cn.fujitsu.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
