arch: remove BR2_arm10t

The BR2_arm10t option is not correct as it references an ARM family,
while other options indicate a specific ARM core. The ARM cores in
ARM10 family are ARM1020E, ARM1022E and ARM1026EJ-S according to
Wikipedia. However, those are clearly very rare, and Wikipedia only
indicates two Conexant ADSL-related SoC as being part of this family
of ARM cores. Therefore, this commit removes this ARM family.

[Peter: remove nettle.mk reference as pointed out by Yann]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>