Remove 3.0 migration guide

Migrating from 2.x to 3.0 is no longer relevant: some of the advice wouldn't
work with 4.0. We don't really need a migration guide from 3.x any longer
since 2.x is no longer officially supported.

Signed-off-by: Gilles Peskine <[email protected]>

Author: gilles-peskine-arm