The GN build now requires that users install depot_tools to their path, rendering our vendored version obsolete.

* build: remove gyp build files

* docs: update build instructions

* build: temporary restore electron.gyp

* build: do not update Electron version in the electron.gyp

* chore: remove unused submodules

* ci: remove obsolete CI scripts and configs

* chore: remove obsolete scripts

* chore: remove obsolete functions from lib/util.py

* ci: send Slack notification for nightly build results

your refs)

your ref)

Crashpad use `gclient` to maintain its third_party libraries.
This patch switches `gclient` instead of `git submodule` to align crashpad way.