do not index links in core repository

[tomkralidis]

Overview

This PR removes indexing for links in the core repository. The following commands were run on each of the *.db files in this PR:

drop index ix_records_links;
vacuum;

Related Issue / Discussion

None

Additional Information

None

Contributions and Licensing

(as per https://github.com/geopython/pycsw/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pycsw. I confirm that my contributions to pycsw will be compatible with the pycsw license guidelines at the time of contribution.
• I have already previously agreed to the pycsw Contributions and Licensing Guidelines