Bug 31537: Elasticsearch - index mapping for 003 control-number-identifier is twice...

Bug 31537: Elasticsearch - index mapping for 003 control-number-identifier is twice in mappings.yaml

This patch removes one of the two mappings for the 003 field to the
control-number-identifier index (for MARC21).

To test:
1) Apply patch
2) reindex with mappings reset
3) try to search for cni:code (for example cni:OSt)
--> it should return the desired results
4) try to search for control-number-identifier:code (for example
control-number-identifier:OSt)
--> it should return the desired results
5) Optionally, try the test plan in Bug 11175 to make sure it still
works

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>