Remove duplicate entry from all_models.yml

This line is already present further down in the YAML file. In EE the
line is already removed.

spec/lib/gitlab/import_export/all_models.yml

@@ -343,7 +343,6 @@ project:
 - fork_network_member
 - fork_network
 - custom_attributes
-- prometheus_metrics
 - lfs_file_locks
 - project_badges
 - source_of_merge_requests