arc/bfin: remove 60x cores

arc/bfin: remove 60x cores

Those cores are not supported in upstream gcc, not even in master.

The only toolchain that supported those core was the 2014R1 ADI
rebuilt toolchain, but we removed it in 311bc13 (toolchain: kill
ADI Blackfin toolchain) because there was too many issues with it.
ADI has not released any newer toolchain since then.

There is little hope for those cores now, so remove them.

Support for those cores has been useless and unusable for a while
without nobody noticing, therefore we intentionally skip adding
Config.in.legacy. This would require keeping code in
arch/Config.in.bfin since the options being removed are inside a
choice...endchoice block.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas: explain why we don't add the options to Config.in.legacy.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>