openthread: remove upstreamed OpenThread recipes from staging

These recipes have been upstreamed to meta-networking and are included
in the latest revision we use of it. No need to keep a copy around here.

https://git.openembedded.org/meta-openembedded/commit/?id=bb6a70a4638688ca971dc281d90627135f59ce0c
https://git.openembedded.org/meta-openembedded/commit/?id=47640c2235ebfca8abfa1524087893e1ece095d5
https://git.openembedded.org/meta-openembedded/commit/?id=b75d3b24618c735571e5d53f2574fefa3e75ce36



Signed-off-by: Stefan Schmidt <stefan.schmidt@huawei.com>