Remove Unicorn support: docs

README.md

doc/administration/database_load_balancing.md

doc/administration/geo/replication/geo_validation_tests.md

doc/administration/geo/replication/multiple_servers.md

doc/administration/gitaly/configure_gitaly.md

doc/administration/gitaly/index.md

doc/administration/logs.md

doc/administration/monitoring/prometheus/index.md

doc/administration/nfs.md

doc/administration/operations/fast_ssh_key_lookup.md

doc/administration/operations/puma.md

doc/administration/read_only_gitlab.md

doc/administration/reference_architectures/10k_users.md

doc/administration/reference_architectures/25k_users.md

doc/administration/reference_architectures/2k_users.md

doc/administration/reference_architectures/3k_users.md

doc/administration/reference_architectures/50k_users.md

doc/administration/reference_architectures/5k_users.md

doc/administration/troubleshooting/debug.md

doc/administration/troubleshooting/linux_cheat_sheet.md