This reverts:
3fb4140c lua/defaults: add user_data helpers
68a20e7a javascript/defaults: add user_data helpers
00510379 lua/js: fix user_data_del util function
As well as the lua/js parts of:
3ec2a098 docs: document new user-data property
user-data and its sub-properties can be set/get/observed/deleted
via the standard properties interface, so there's no need for
additional helpers specific to user-data, which only added maintenance
burden.