lib: kunit: Fix compilation test when using TEST_BIT_FIELD_COMPILE

A build condition was missing around a compilation test, this compilation
test comes from the original test_bitfield code.

And removed unnecessary code for this test.

Fixes: d2585f51 ("lib: kunit: add bitfield test conversion to KUnit")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Vitor Massaru Iha <vitor@massaru.org>
Link: https://lore.kernel.org/linux-next/20201015163056.56fcc835@canb.auug.org.au/


Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>