Protect $g_default_category_for_moves from deletion

When a category defined as default_category_for_moves is deleted and an
issue is subsequently moved to another project where its current
category does not exist, it gets assigned a non-existing category. This
causes application error 1502 to be triggered whenever MantisBT tries to
display the issue's Category, which can cause a system lock up.

This commit reduces the risk of this situation from happening, by
preventing users from deleting categories which are used as default
(either defined in config_inc.php or in the config table).

Fixes 0014478