Remove unused ProjectPushRule model

What does this MR do and why?

Removes the unused ProjectPushRule model and its supporting pieces:

• ee/app/models/project_push_rule.rb
• the has_one :project_push_rule association in ee/app/models/ee/project.rb
• the :project_push_rule factory and model spec
• the association spec in ee/spec/models/ee/project_spec.rb
• the project_push_rule entries in spec/lib/gitlab/import_export/all_models.yml
• the corresponding .rubocop_todo entries

ProjectPushRule was introduced for the read switch to the project_push_rules table, which was later reverted in #592187 (closed). Nothing instantiates or reads the model since then. The project_push_rules table itself will be dropped in a follow-up MR with a post-deployment migration.

This is the second of several small cleanup MRs for #605248 (closed), following !250396 (merged).

References

• Issue: #605248 (closed)
• Parent tracking issue: #499156
• Read switch revert: #592187 (closed)
• Previous cleanup MR: !250396 (merged)

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