Revert "autotest: tko: connect using unix socket"

This reverts commit f8046bb3d4523676aef46e8a15ddb5ef850bf7b5.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> autotest: tko: connect using unix socket
> 
> In a previous change, https://chromium-review.googlesource.com/1164064,
> we added a new configuration in shadow_config.ini, global_db_socket, and
> return it as a new entry in the settings dict. It works well, however,
> it breaks code using django.
> 
> Django only uses settings of 'HOST'. It can be either a real host name,
> or a unix socket if it starts with '/'.
> 
> BUG=chromium:868052
> TEST=Ran below code in python interpreter on guocb-dev-autotest2.cbf
> 
>     1) Verified TKO connection works:
>     import common
>     from autotest_lib.tko import db
>     sql = db.db()
> 
>     2) Also verified the Django connection works.
> 
>     import common
>     import os
>     os.environ.setdefault('DJANGO_SETTINGS_MODULE',
>                           'autotest_lib.frontend.settings')
>     from dango.db import connections
> 
>     c = connections['global'].cursor()
> 
> Change-Id: I4bca254ae63591536a900457ebcfbc71493af00e
> Reviewed-on: https://chromium-review.googlesource.com/1213781
> Commit-Ready: Congbin Guo <guocb@chromium.org>
> Tested-by: Congbin Guo <guocb@chromium.org>
> Reviewed-by: Congbin Guo <guocb@chromium.org>

Bug: chromium:868052
Change-Id: I302b2eed06d55dd23cf8ca3ff9e76c6c4f8eafff
Reviewed-on: https://chromium-review.googlesource.com/1226152
Reviewed-by: Allen Li <ayatane@chromium.org>
Tested-by: Allen Li <ayatane@chromium.org>
diff --git a/frontend/database_settings_helper.py b/frontend/database_settings_helper.py
index 875b27d..dd7a84c 100644
--- a/frontend/database_settings_helper.py
+++ b/frontend/database_settings_helper.py
@@ -139,12 +139,11 @@
     config = {
         'ENGINE': 'autotest_lib.frontend.db.backends.afe',
         'PORT': getter('port', default=''),
-        # Django use setting of HOST for both real host and unix socket. See
-        # crbug.com/868052 for details.
-        'HOST': getter('socket', default=None) or getter('host'),
+        'HOST': getter('host'),
         'NAME': getter('database'),
         'USER': getter('user'),
         'PASSWORD': getter('password', default=''),
+        'PROXY_SOCKET': getter('socket', default=None),
         'READONLY_HOST': getter('readonly_host', default=getter('host')),
         'READONLY_USER': getter('readonly_user', default=getter('user')),
     }