Bug 10580: adjust Makefile.PL to reflect that Zebra no longer optional in Koha setup

Running Makefile.PL asks the user whether to install the Zebra
configuration files and different texts relate to the user having
chosen to use Zebra.

This patch removes references to choosing to use Zebra and removes
the related variables from the code.

To test:
- Apply the patch
- Go through the different install modes
- Verify that nothing nothing is broken

Sponsored-by: Universidad Nacional de Cordoba
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Bug 10580: (follow-up) tweak wording about looking for zebrasrv & zebraidx

Signed-off-by: Galen Charlton <gmc@esilibrary.com>