ci: Remove duplicate RSpec jobs

What does this MR do and why?

We have some duplicated jobs. Following is the explanation:

rspec-ee unit pg12:
  extends:
    - .rspec-ee-base-pg12
    - .rails:rules:ee-only-unit
    - .rspec-ee-unit-parallel

# which is equivalent to the following after replacing `.rspec-ee-base-pg12` by its definition

rspec-ee unit pg12:
  extends:
    - .rspec-base
    - .use-pg12-es7-ee
    - .rails:rules:ee-only-unit
    - .rspec-ee-unit-parallel

Then if we do the same thing for rspec-ee unit pg12 es7:

rspec-ee unit pg12 es7:
  extends:
    - .rspec-ee-base-pg12-es7
    - .rspec-ee-unit-parallel
    - .rails:rules:default-branch-schedule-nightly--code-backstage-ee-only

# which is equivalent to the following after replacing `.rspec-ee-base-pg12` by its definition

rspec-ee unit pg12 es7:
  extends:
    - .rspec-base
    - .use-pg12-es7-ee
    - .rails:rules:run-search-tests
    - .rspec-ee-unit-parallel
    - .rails:rules:default-branch-schedule-nightly--code-backstage-ee-only

# .rails:rules:run-search-tests is never used since it's overridden by .rails:rules:default-branch-schedule-nightly--code-backstage-ee-only, so we have:

rspec-ee unit pg12 es7:
  extends:
    - .rspec-base
    - .use-pg12-es7-ee
    - .rspec-ee-unit-parallel
    - .rails:rules:default-branch-schedule-nightly--code-backstage-ee-only

# which is quite similar to 

rspec-ee unit pg12:
  extends:
    - .rspec-base
    - .use-pg12-es7-ee
    - .rails:rules:ee-only-unit
    - .rspec-ee-unit-parallel

Since .rails:rules:default-branch-schedule-nightly--code-backstage-ee-only is:

.rails:rules:default-branch-schedule-nightly--code-backstage-ee-only:
  rules:
    - <<: *if-not-ee
      when: never
    - !reference [".rails:rules:default-branch-schedule-nightly--code-backstage-default-rules", rules]

# which is equivalent to the following after replacing `!reference [".rails:rules:default-branch-schedule-nightly--code-backstage-default-rules", rules]` by its definition

.rails:rules:default-branch-schedule-nightly--code-backstage-ee-only:
  rules:
    - <<: *if-not-ee
      when: never
    - <<: *if-default-branch-schedule-nightly
    - <<: *if-merge-request
      changes: [".gitlab/ci/rails.gitlab-ci.yml"]

Here we can ignore

    - <<: *if-merge-request
      changes: [".gitlab/ci/rails.gitlab-ci.yml"]

since it's covered by .rails:rules:ee-only-unit already, which leave us with <<: *if-default-branch-schedule-nightly, which is also covered by .rails:rules:ee-only-unit with the following rule:

    - <<: *if-default-refs
      changes: *backend-patterns

since changes is always evaluated to true for scheduled pipelines, and if-default-refs includes scheduled pipelines (since it runs for master).

The same logic applies for rspec-ee integration pg12 es7, and rspec-ee system pg12 es7.

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