cassini-[docs,tests]: Remove `root` account tests

Current LAVA test framework (inline test action) needs to run
`transfer_overlay` action if there are any shell commands in the
inline test action. In order for `transfer_overlay` to work `root`
account login is required because, overlay is done at `/`. Therefore,
do not disable `root` account when `cassini-security` and
`cassini-test` distro features are enabled.

Since the `root` account login is enabled when `cassini-test` distro
feature is enabled, remove `root` account tests.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>