build: remove gyp build files (#14097)

* 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

script/bump-version.py

@@ -116,6 +116,8 @@ def increase_version(versions, index):
 
 
 def update_electron_gyp(version, suffix):
+  assert(False, "electron.gyp must not be used anymore. We build with GN now.")
+
   pattern = re.compile(" *'version%' *: *'[0-9.]+(-beta[0-9.]*)?(-dev)?"
     + "(-nightly[0-9.]*)?'")
   with open('electron.gyp', 'r') as f: