selftests/lkdtm: Remove dead config option

CONFIG_HARDENED_USERCOPY_FALLBACK config option has been removed in
commit 53944f17 ("mm: remove HARDENED_USERCOPY_FALLBACK"). Remove it
from the lkdtm selftest config.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>