Bug 36793: Fix case of $var in Context->delete_preference

Bug 36793: Fix case of $var in Context->delete_preference

This only applies to 'local' preferences.

Test plan (first without this patch):
Add a local pref. Delete it. Ask value on commandline with:
* perl -MC4::Context -e"print C4::Context->preference('YOUR_PREF')"
* Did you replace YOUR_PREF :)

Now repeat with this patch. And verify fix (no value now).

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit 8ea126b1f89e24a2baf8c43f7bf837bf74741088)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>