PR template: remove that changelog extension is requested twice

[smelc]

Changelog

- description: |
    Remove doublon in PR template
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  - improvement    # QoL changes e.g. refactoring
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

Like IntersectMBO/cardano-api#285, but for cardano-cli this time

As visible in this PR, the Checklist section has a doublon regarding checking that the PR changelog is specified, it contains both:

The change log section in the PR description has been filled in

and

The changelog section in the PR is updated to describe the change

This PR removes the second entry.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• The change log section in the PR description has been filled in
• NA New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • round trip tests
  • integration tests
    See Running tests for more details
• NA The version bounds in .cabal files are updated
• CI passes. See note on CI. The following CI checks are required:ghc-9.2.7
• The changelog section in the PR is updated to describe the change
• Self-reviewed the diff