Delete .github, .gitmodules and BUILD files

Those files are causing noise and fails the linter, .gitmodules would
trigger sub-git repository linters and BUILD files are still recognized
by AOSP (roboleaf).

See https://android-review.git.corp.google.com/c/platform/external/cronet/+/3278451/1
for the approved CL

Change-Id: I9371212d5ba74a311962d97fbec9eb595447a90c
84 files changed