Bug 33431: Make C4::Circulation use C4::Context->yaml_preference

Bug 33431: Make C4::Circulation use C4::Context->yaml_preference

This patch removes manual YAML handling for sysprefs in C4::Circulation.

It also makes C4::Context->yaml_preference not warn when undef is
retrieved from the sysprefs.

To test:
1. Run:
   $ ktd --shell
  k$ prove t/db_dependent/Circulation*
=> SUCCESS: Tests pass!
2. Apply this patch
3. Repeat 1
=> SUCCESS: Tests pass!
4. Sign off :-D

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>