Remove code related to legacy ingestion support

[aditya1702]

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

Removes configuration flags (and related tests) supporting legacy ingestion:

• stellar-core-db-url
• enable-captive-core-ingestion

Note: The flag definitions are still present and a deprecation warning is added to the command-line.

Why

As part of #4855, we want to remove support for legacy ingestion from Horizon.

• Ops team has successfully shut down all legacy ingestors on public Horizon - https://github.com/stellar/ops/issues/2557
• As part of partner experience revamp, we have removed all references of above config variables from docs - Partner Experience: Update Horizon documentation stellar-docs#249

Known limitations

N/A

[sreuland]

nice work!