Documentation: Remove the unused "tp720" from kernel-parameters.txt

The "tp720" switch once belonged to the ps2esdi driver, but this
driver has been removed a long time ago in 2008 in the commit
2af3e601 ("The ps2esdi driver was marked as BROKEN more than two years ago due to being no longer working for some time.")
already, so let's remove it from the documentation now, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240617073322.40679-1-thuth@redhat.com