lua: drop luaL_register_module()

The function was replaced by luaT_newmodule(). See commit e3cf5a5d
("lua: add built-in module registration function") for the motivation
and details.

Now all luaL_register_module() usages are eliminated (including
Tarantool Enterprise source code) and we can safely drop it.

Part of #7774
Follows up PR #8173

NO_DOC=no user visible changes: the function is internal
NO_TEST=see NO_DOC
NO_CHANGELOG=see NO_DOC