toolchain-external: remove CS sh2 toolchains

Normally we'd deprecate them, but:

1) They don't support IPv6 and it's being removed so it makes no sense.
2) They're based on uClibc 0.9.30-ish which is very old and surely has
package build breakage all over it.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>