docs: kselftest: remove link to obsolete wiki

docs: kselftest: remove link to obsolete wiki

Remove the reference to the obsolete kselftest wiki.

The kselftest wiki is marked obsolete and is no longer updated. The last
edit was in 2019, and the information is outdated, referring readers for
support to IRC networks that have not been used for years, and to kernel
versions that are no longer supported.

If there is any relevant information left in the wiki it needs to be
cleaned up and moved into the canonical kselftest documentation here.

Link: https://lore.kernel.org/r/20260115172817.7120-1-bacs@librecast.net
Signed-off-by: Brett A C Sheffield <bacs@librecast.net>
Acked-by: Bagas Sanjaya <bagasdotme@gmail.com>
Acked-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>