Documentation: kunit: Remove redundant 'tips.rst' page

The contents of 'tips.rst' was mostly included in 'usage.rst' way back in
commit 95357439 ("Documentation: KUnit: Rework writing page to focus on writing tests"),
but the tips page remained behind as well.

The parent patches in this series fill in the gaps, so now 'tips.rst' is
redundant.
Therefore, delete 'tips.rst'.

While I regret breaking any links to 'tips' which might exist
externally, it's confusing to have two subtly different versions of the
same content around.

Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Sadiya Kazi <sadiyakazi@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>