Remove CASTOR to CTA migration tools RPM

The tools under CTA/migration that were used for the CASTOR to CTA migration are no longer needed and should be removed from the RPMs and the CTA repo.

This involves the following steps:

• Remove cta-migration-tools from versionlock.cta and ctaeos-mgm.sh
• Remove the package cta-migration-tools from cta.spec.in
• Remove add_subdirectory(migration) from top-level CMakeLists.txt
• Remove castor.repo from CI

The tools can then be moved to a separate repo (see https://gitlab.cern.ch/cta/CTA/-/issues/1182)