Bug 12538: Remove Solr without breaking anything else

Bug 12538: Remove Solr without breaking anything else

Since nobody is currently working on the zebra layer introduced by bug
8233, Solr won't never work.
Some code has been introduced in 3.10 to prove several search engines
can cohabit into Koha but no help/fund has been found to go ahead.
It is useless to keep this code and to maintain an ambiguous situation.

I think the indexes configuration page could be restore later if someone
else introduces a new search engine into Koha.

Test plan:
Look at the code introduced by bug 8233 and verify all is removed.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>