Remove AmigaOneG3SE board

The AmigaOneG3SE board has been orphaned or a very long time, and
broken for more than 12 releases resp. more than 3 years.  As nobody
seems to be interested any more in this stuff we may as well ged rid
of it, especially as it clutters many areas of the code so it is a
continuous pain for all kinds of ongoing work.

Signed-off-by: Wolfgang Denk <wd@denx.de>