Documentation: kunit: Remove references to --defconfig

The --defconfig option in kunit_tool was removed in [1], but the getting
started and kunit_tool documentation still encouraged its use. Update
those documents to reflect that it's no-longer required, and is the
default behaviour if no .kunitconfig is found.

Also update a couple of places where .kunitconfig is still referred to
as kunitconfig (this was changed in [2]).

[1]:
https://git.kernel.org/pub/scm/linux/kernel/git/shuah/linux-kselftest.git/commit/?h=kunit-fixes&id=9bdf64b35117cc10813d24e1842cd8ee40ecbf19
[2]:
https://git.kernel.org/pub/scm/linux/kernel/git/shuah/linux-kselftest.git/commit/?h=kunit-fixes&id=14ee5cfd4512ee3d1e0047d8751450dcc6544070



Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>