package/qemu: remove csky fork

package/qemu: remove csky fork

We have a qemu fork for csky cpus [1] but since qemu version
bump to 4.2.0 [2] and libssh2/libssh change the csky build is
broken.

The csky fork is based on Qemu 3.0.0 but unlike autotools packages
any unknown option is handled as error.

Since we don't want to support all options from previous qemu
release and the github repository has been removed [3] and the
only remaining archive is located on http://sources.buildroot.net,
remove the qemu csky fork as suggested by [4].

[1] https://git.buildroot.net/buildroot/commit/?id=f816e5b276f1ef15840bec6667f1e8219717ab7d
[2] https://git.buildroot.net/buildroot/commit/?id=0ea17054ce7dfc54efca5634133cef786445e7b1
[3] https://github.com/c-sky/qemu
[4] http://lists.busybox.net/pipermail/buildroot/2020-May/281885.html

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Guo Ren <ren_guo@c-sky.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
[Peter: move patches out of 4.2.0 subdir]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>