services/horizon: Remove the optimization that uses COPY for liquidity pool insertion

[urvisavla]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

Remove the optimization that uses COPY for liquidity pool insertion and revert to the previous method.

Why

Fixes #5134

To improve liquidity pool data insert performance, the COPY command was introduced to perform bulk insertions (#5110). This optimization assumed only newly created liquidity pools would be inserted using the COPY method, preventing conflicts with existing records. However, when the final trust-line for a liquidity pool is removed, the corresponding liquidity pool record is marked for later deletion. And if a new trust-line is added before the actual deletion, inserting the updated liquidity pool data using COPY fails due to the duplicate record.

The solution is to revert to the original upsert method for liquidity pool insertions which handles both new insertions and updates to existing records.

Known limitations

N/A