Remove autostart commands after they have been executed.

Remove autostart commands after they have been executed.

As there is no need to keep autostart commands in memory, we can safely
remove them as soon as they have been executed. As we previously didn't
clear them at all during config reloads, this also fixes a memory leak.

Note that neither autostarts nor autostarts_always is executed during
config reloads, so removing them from memory is fine as an i3 restart
will cause them to be parsed again.

fixes #2044