Free ran_assignments

Free ran_assignments

When we run 'reload' all the assignments are freed:
https://github.com/i3/i3/blob/e3e09119bf994ea3f5222441832952a8dd352941/src/config.c#L99-L109

Assignments are saved to each window after they are executed:
https://github.com/i3/i3/blob/e3e09119bf994ea3f5222441832952a8dd352941/src/assignments.c#L41-L46

This means that the pointers stored in window->ran_assignments are
invalid (shouldn't be dangerous currently but could lead to a segfault
if the code is modified) after a 'reload'.