Eliminate hard-coded "user settings" menu (in useredit.cpp)

sbbs_t::user_config() should just execute the configured loadable module for this functionality and nothing more.

Having multiple implementations of the user (default) config/settings menu can be confusing to support. As of v3.20, user_settings.js was included and enabled as the usercfg_mod by default for new installs, but now's the time to force this script as the default usercfg_mod value (for those upgrading from v3.19 and earlier) and eliminate the hard-coded menu.