altera_*_defconfig: remove broken defconfigs

altera_*_defconfig: remove broken defconfigs

These defconfigs have been broken due to gcc5 for more than half a year,
and nobody stepped up to fix it. In addition, the rocketboards git
repository used by these defconfigs hasn't been reachable for more than
half a year. Finally, these defconfigs don't even use the
BR2_TARGET_UBOOT_ALTERA_SOCFPGA_IMAGE_CRC option so they are not a
great example for this family of targets.

Remove these defconfigs, as well as the board directory. Both are
removed in a single commit to avoid having an inconsistent readme.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Jan Viktorin <viktorin@rehivetech.com>
Cc: Charles Manning <cdhmanning@gmail.com>
Cc: Sebastien Bourdelin <sebastien.bourdelin@savoirfairelinux.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>