Bug 14302: Remove GRS1 specific code

Bug 14302: Remove GRS1 specific code

Remove:
- BIB_INDEX_MODE and AUTH_INDEX_MODE env var
- bib_index_mode and auth_index_mode options from scripts
- Warnings from about page, just kept one if zebra_bib_index_mode or
zebra_auth_index_mode still exist in config and are set to grs1

Test plan:
- Install Koha from src
- Install Koha from pkg
- Read the code, carefully!

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Rebased

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>