conf: machine: remove ext3 from MACHINE_FEATURES

ext3 no longer exists as a machine feature.
Only "ext2" does (used in meta/recipes-core/packagegroups/packagegroup-base.bb)

Signed-off-by: Michael Opdenacker <michael.opdenacker@rootcommit.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>