Remove support for the deprecated libreload command

As mentioned in the ARM:

The libreload command was deprecated in Kea 2.3.4. The code to handle this command is still there, but there are reports of it being buggy and not really usable. Kea 2.3 and 2.4 versions will produce a warning when this command is used, and it will be removed entirely sometime in the 2.5 branch.