gluon-core: remove obsolete gluon.site_config library

package/gluon-core/luasrc/usr/lib/lua/gluon/site_config.lua

-local site = require 'gluon.site'
-
-local setmetatable = setmetatable
-
-module 'gluon.site_config'
-
-setmetatable(_M, {
-	__index = site(),
-})
-
-return _M