remove the Thread Network HAL from the stage folder

The Thread Network HAL is going to be added to the AOSP. The
implementation of the Thread Network HAL under the staging
folder is no longer needed. This commit removes the unstable
Thread Network HAL.

Bug: 283905423
Test: Build
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:b044d98115f19fa6d1e430fd6065a372153f8680)
Merged-In: I7e4bf260c03dd0500725fec068747e3b3d05820c

Change-Id: I8c7ea027b45511597c4014785718b4bda4198520