Bug 9612: (follow-up) restore elementSetName in Context.pm

Bug 9612: (follow-up) restore elementSetName in Context.pm

Restore elementSetName to marcxml for DOM indexing in Zconn (Context.pm).
This prevents the need of rebuilding the index after restarting Zebra
server.

Removes the now incorrect reference to marcxml as 'superfluous' in four
dom config files.

Test plan:
[1] Do not yet apply this patch.
[2] Rebuild zebra index with the zebra config of commit
    036f2a50e11dab97ef8509f2f585c4cc407ea728.
[3] (Go back to master.) Restart your zebra server (no config change).
    You will have results without details.
    Apply this patch: you see details.
    Reset to master: no details again.
[4] Install new zebra config from master.
    Search again: you still see no details.
    Restart zebra server. Search: you see details.
    Apply this patch. Search: still details.
    Restart zebra server. Search: still details.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Tested in a non-package environment (manual dev install).
The package environment should work now too (results in step 4c might differ).
Progress on bug 12012 would be appropriate to sync all changes.
Tested the response of the SRU server too.

Signed-off-by: Marc Veron <veron@veron.ch>
I tested starting on a VM with Koha 3.15.00.019 installed.
Did git pull -> Koha 3.15.00.051
Result: No details in search results.
Applied patch.
Result: Search results display fine.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>