chore(migrations) remove deprecated Cassandra migrations helpers (#8781)

### Summary

This library had only one function left uncommented and that
was about `Cassandra`. As `Cassandra` is deprecated with upcoming
`3.0.0`, we don't use / need this anymore, thus removing it.

CHANGELOG.md

@@ -99,6 +99,9 @@
   [#8552](https://github.com/Kong/kong/pull/8552). If you are using
   [Go plugin server](https://github.com/Kong/go-pluginserver), please migrate your plugins to use the
   [Go PDK](https://github.com/Kong/go-pdk) before upgrading.
+- The migration helper library is no longer supplied with Kong (we didn't use it for anything,
+  and the only function it had, was for the deprecated Cassandra).
+  [#8781](https://github.com/Kong/kong/pull/8781)
 
 
 #### Plugins

kong-2.8.0-0.rockspec

@@ -214,7 +214,6 @@ build = {
     ["kong.db.strategies.off.tags"] = "kong/db/strategies/off/tags.lua",
 
     ["kong.db.migrations.state"] = "kong/db/migrations/state.lua",
-    ["kong.db.migrations.helpers"] = "kong/db/migrations/helpers.lua",
     ["kong.db.migrations.subsystems"] = "kong/db/migrations/subsystems.lua",
     ["kong.db.migrations.core"] = "kong/db/migrations/core/init.lua",
     ["kong.db.migrations.core.000_base"] = "kong/db/migrations/core/000_base.lua",

kong/db/init.lua

@@ -420,7 +420,6 @@ end
 do
   -- migrations
   local utils = require "kong.tools.utils"
-  local MigrationHelpers = require "kong.db.migrations.helpers"
   local MigrationsState = require "kong.db.migrations.state"
 
 
@@ -517,8 +516,6 @@ do
       return nil, prefix_err(self, err)
     end
 
-    local mig_helpers = MigrationHelpers.new(self.connector)
-
     local n_migrations = 0
     local n_pending = 0
 
@@ -558,9 +555,7 @@ do
           end
 
           if strategy_migration.up_f then
-            local pok, perr, err = xpcall(strategy_migration.up_f,
-                                          debug.traceback, self.connector,
-                                          mig_helpers)
+            local pok, perr, err = xpcall(strategy_migration.up_f, debug.traceback, self.connector)
             if not pok or err then
               self.connector:close()
               return nil, fmt_err(self, "failed to run migration '%s' up_f: %s",
@@ -601,8 +596,7 @@ do
           -- kong migrations teardown
           local f = strategy_migration.teardown
 
-          local pok, perr, err = xpcall(f, debug.traceback, self.connector,
-                                        mig_helpers)
+          local pok, perr, err = xpcall(f, debug.traceback, self.connector)
           if not pok or err then
             self.connector:close()
             return nil, fmt_err(self, "failed to run migration '%s' teardown: %s",