Documentation: kunit: remove claims that kunit is a mocking framework

KUnit does not have any first party support for "mocking".

The original RFC had some, but the code got dropped.
However, the documentation patches never got updated. This fixes that.

https://kunit.dev/mocking.html

 has a current writeup on the status quo
and will hopefully be eventually folded into the in-kernel
Documentation.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>