[docker] Remove mention of pre-built container
[docker] Remove mention of pre-built container

The pre-built container has last been updated in 2020. Unless we start
automatically updating this container, it'll only guide users to an
outdated and unsupported configuration, which is worse than having to
build container by hand.

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan