Bug 32478: Remove Koha::Config::SysPref->find since bypasses cache

Bug 32478: Remove Koha::Config::SysPref->find since bypasses cache

get_yaml_pref_hash also allows invalid YAML and only parses a limited
subset so remove this method to avoid future issues.

To test):

Since tests already exists for C4::Context->yaml_preference and this
is a trivial change, do we really need a test plan for this?

Sponsored-by: Gothenburg University Library
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>