Variable `target->semihosting` is always initialized
in risv_init_target() to a non-NULL value.
For that reason, checks like `if (target->semihosting)`
are redundant (dead code).
Remove them to not confuse code readers. Replace them by assertions.
Change-Id: I85ef52300e240cfcb0119db6169993bc4767de8f
Signed-off-by: Jan Matyas <jan.matyas@codasip.com>