Change down migrations to be empty files

They had a simple `SELECT 1` before because that's what we've done in
other instances as we had to support MySQL. Postgres is able to
process empty migrations, so as the plugin only supports running on
Postgres, the files are now empty.

server/store/sqlstore/migrations/000001_system_settings_table.down.sql

@@ -1 +0,0 @@
-SELECT 1;

server/store/sqlstore/migrations/000002_subscriptions_table.down.sql

@@ -1 +0,0 @@
-SELECT 1;

server/store/sqlstore/migrations/000003_links_table.down.sql

@@ -1 +0,0 @@
-SELECT 1;

server/store/sqlstore/migrations/000004_users_table.down.sql

@@ -1 +0,0 @@
-SELECT 1;

server/store/sqlstore/migrations/000005_posts_table.down.sql

@@ -1 +0,0 @@
-SELECT 1;

server/store/sqlstore/migrations/000006_links_teamid_channelid_index.down.sql

@@ -1 +0,0 @@
-SELECT 1;

server/store/sqlstore/migrations/000007_users_userid_index.down.sql

@@ -1 +0,0 @@
-SELECT 1;

server/store/sqlstore/migrations/000008_posts_channelid_postid_index.down.sql

@@ -1 +0,0 @@
-SELECT 1;

server/store/sqlstore/migrations/000009_subscriptions_add_certificate_and_lastactivityat_columns.down.sql

@@ -1 +0,0 @@
-SELECT 1;

server/store/sqlstore/migrations/000010_invited_users_table.down.sql

@@ -1 +0,0 @@
-SELECT 1;

server/store/sqlstore/migrations/000011_users_add_lastconnectat_lastdisconnectat_columns.down.sql

@@ -1 +0,0 @@
-SELECT 1;

server/store/sqlstore/migrations/000012_whitelist_table.down.sql

@@ -1 +0,0 @@
-SELECT 1;

server/store/sqlstore/migrations/000013_users_teamsuserid_unique_index.down.sql

@@ -1 +0,0 @@
-SELECT 1;

server/store/sqlstore/migrations/000014_delete_whitelisted_users_table.down.sql

@@ -1 +0,0 @@
-SELECT 1;

server/store/sqlstore/migrations/000015_users_add_lastchatsentat_lastchatreceivedat_columns.down.sql

@@ -1 +0,0 @@
-SELECT 1;