[electron] remove look up from user home dir

[akosyakov]

This code should be deleted:

theia-apps/theia-electron/scripts/theia-electron-main.js

Lines 7 to 10 in 140a9a1

	// Lookup inside the user's home folder for more plugins, and accept user-defined paths.
	process.env.THEIA_PLUGINS = [
	process.env.THEIA_PLUGINS, `local-dir:${path.resolve(os.homedir(), '.theia', 'plugins')}`,
	].filter(Boolean).join(',')

Review other electron apps for it as well.

Since eclipse-theia/theia@c3e162c Theia automatically loads VS Code extensions from user config dir and install them there.

[akosyakov]

It has to be done after #325

[marcdumais-work]

@lmcbout Now that the images we have as part of the CI have been updated to include @theia/vsx-registry, we can proceed with this change. Can you take care of it?