initscripts: Remove /etc/volatile.cache on upgrade

/etc/volatile.cache is a cached copy of a script (which is
generated by /etc/init.d/populate-volatile.sh) that generates
the volatile filesystem directories.  Since volatile.cache is
a generated file, it is not necessarily changed if
populate-volatile.sh is updated.  As a result, the stale script
can add/remove the wrong directories on the next system boot.

If initscripts is being upgraded, make sure volatile.cache gets
deleted.

Signed-off-by: Bryan Evenson <bevenson@melinkcorp.com>