Bug 18045: Fix Reserves.t tests - caching issues

Bug 18045: Fix Reserves.t tests - caching issues

If the cache has to be cleaned when marc structure is updated.

Before this patch, if the age restriction was not mapped, Reserves.t
failed with:
  Tag "" is not a valid tag. at t/db_dependent/Reserves.t line 523.

Test plan:
Just confirm that the tests still pass and that the changes make sense.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
It seems that only removing the key MarcSubfieldStructure- should be
enough, but it does not hurt either to remove those four keys.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>